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STRUCTURE  AND  DEVELOPMENT  OF 
SPORANGIA  AND  SPOROPHYLLS 
OF  ISOETES 


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SCHOOLS  OF  ARTS,  LITERATURE,  AND  SCIENCE,  IN  CANDIDACY 
FOR  THE  DEGREE  OF  DOCTOR  OF  PHILOSOPHY 

DEPARTMENT  OF  BOTANY 


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1900 


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VOLUME  XXIX 


NUMBER  4 


Botanical  Gazette 


APRIL ,  igoo 


THE  STRUCTURE  AND  DEVELOPMENT  OF  THE 
SPOROPHYLLS  AND  SPORANGIA  OF  ISOETES. 

CONTRIBUTION  FROM  THE  HULL  BOTANICAL  LABORATORY. 


XVIII. 

R.  Wilson  Smith, 
(with  plates  xiii-xx) 


Few  plants  have  excited  more  interest  than  Isoetes,  a  small 
genus  of  about  fifty  species,  which  has  been  variously  classified, 
and  the  histology  and  development  of  which  have  been  described 
in  the  most  contradictory  manner.  It  was  with  the  purpose  of 
obtaining,  if  possible,  some  data  by  which  to  clear  up  its  homol¬ 
ogies  and  relationships,  and  especially  of  examining  the  founda¬ 
tion  of  a  claim  made  in  recent  years  of  its  being  the  point  of 
contact  between  monocotyledons  and  vascular  cryptogams  that 
the  following  investigation  was  undertaken. 

The  intention  at  first  was  to  have  the  work  include  not  only  the 
reproductive  parts  of  the  sporophyte,  but  also  the  development 
of  the  female  gametophyte  and  of  the  embryo.  But  so  small  a 
proportion  of  the  spores  was  found  capable  of  germination  that 
the  study  of  the  prothallium  had  to  be  abandoned ;  and  my 


226 


BO  TANICAL  GAZE  TTE 


[APRIL 


observations  of  the  embryo  agree  so  closely  with  those  of  Pro¬ 
fessor  Campbell  (4)  that  it  did  not  seem  worth  while  to  publish 
any  drawings.  One  difference  may  be  noticed  here  and  this 
part  of  the  subject  may  be  dismissed  at  once.  Campbell  says 
that  but  three  archegonia  are  formed  at  first,  and  only  in  case 
none  of  these  are  fertilized  do  others  appear.  I  have  found  new 
archegonia  appearing  even  after  three  embryos  had  begun  to 
develop,  two  of  which  had  made  considerable  growth.  It  thus 
appears  that  sometimes  new  archegonia  may  arise  even  after 
fertilization. 

The  species  selected  for  study  were  /.  echinospora  and  /. 
Engelmanni,  the  former  of  which  was  examined  more  carefully. 
The  material  was  collected  by  Mr.  Raynal  Dodge,  of  Newbury- 
port,  Mass.  Part  of  it  was  fixed  at  once,  and  part  after  it  had 
been  cultivated  for  some  time  in  the  laboratory.  The  fixing 
reagents  employed  were  1  per  cent,  chrom-acetic  acid,  and 
Flemming’s  weaker  solution.  After  remaining  twenty-four  hours 
in  one  of  these  fluids  it  was  washed  twenty-four  hours  in  water  and 
transferred  through  graded  alcohols  and  chloroform  or  xylol  to 
paraffin.  The  sections  were  cut  5,  10,  or  15//,  in  thickness  and 
stained  in  the  ordinary  way  on  the  slides.  Delafield’s  haema- 
toxylin  and  erythrosiri,  safranin  and  gentian  violet,  Heiden- 
hain’s  iron-alum-haematoxylin,  and  cyanin  and  erythrosin  were 
all  used  with  good  results  except  in  the  case  of  the  megaspore 
mother  cell. 

THE  STEM. 

The  technique  which  is  best  adapted  to  an  investigation  of 
the  development  of  the  sporangia  is  not  very  suitable  for  an 
examination  of  the  histology  of  the  stem.  Accordingly  I  have 
not  attempted  to  make  an  exhaustive  study  of  the  latter,  or  of 
the  vascular  bundles  of  the  leaf.  Still  the  arrangement  of  the 
stem  tissues  is  so  peculiar  that  a  few  remarks  will  not  be  out  of 
place.  There  is  probably  little  variation  in  this  respect  in  the 
different  species.  /.  echinospora  and  /.  Engelmanni  agree  very 
closely  with  /.  lacustris  as  figured  and  described  by  Farmer  (1), 
whose  account  is  the  latest  and  best  dealing  with  the  structure 


1 900]  SPOROPHYLLS  AND  SPORANGIA  OF  ISOETES  227 

of  the  vegetative  organs.  The  center  of  the  stem  is  occupied 
by  a  mass  of  short  spiral  and  reticulated  tracheids  interspersed 
especially  near  the  periphery  with  less  numerous  parenchymatous 
cells.  The  peripheral  parenchyma  is  not  sufficiently  aggregated 
or  continuous  to  form  a  xylem  sheath  such  as  occurs  in  the 
leaves.  The  xylem  region  is  surrounded  by  a  ring  of  tabular 
cells  of  glistening  white  appearance,  thick-walled  and  empty 
towards  the  center  but  thin-walled  towards  the  outside  and  more 
or  less  banded  with  incomplete  layers  of  starch-containing  cells. 
The  cells  are  arranged  in  pretty  regular  radial  rows,  whether 
examined  in  longitudinal  or  transverse  section.  This  ring  is 
usually  designated  the  prismatic  layer,  and  very  frequently,  after 
Russow  (1),  the  phloem.  Russow  claimed  to  have  traced  a  con¬ 
tinuity  between  the  prismatic  layer  and  the  phloem  of  the  leaf. 
I  have  not  been  able  to  satisfy  myself  of  any  organic  continuity, 
but  even  did  it  exist  it  seems  to  me  very  questionable  whether 
that  would  be  a  sufficient  reason  to  justify  Russow’s  view.  No 
clearly  defined  sieve-tubes,  the  essential  elements  of  the  phloem, 
have  ever  been  found  either  in  the  stem  or  in  the  leaf;  and 
besides  the  inner  cells  of  the  prismatic  zone  are  known  to  become 
secondarily  thickened  and  transformed  into  xylem  tracheids. 
The  cells  marked  0  in  jig.  5  are  in  this  process  of  transformation. 
Does  not  this  indicate,  if  not  a  xylem  character,  at  least  the 
undifferentiated  nature  of  the  prismatic  cells  ?  A  transforma¬ 
tion  of  phloem  into  xylem  would  be,  to  say  the  least,  an  anomaly. 
In  view  of  these  difficulties  to  which  may  be  added  another  — 
the  relation  to  the  cambium  —  it  seems  better  to  drop  this  appli¬ 
cation  of  the  word  phloem  until  its  justification  shall  be  estab¬ 
lished  on  physiological  grounds.  A  small  portion  of  the  prismatic 
layer  is  shown  in  jig.  5. 

Immediately  outside  the  prismatic  layer  and  indistinguishable 
from  it  except  in  the  staining  and  size  of  the  cells,  is  a 'zone  of 
meristem  which  by  its  active  division  gives  origin  outwardly  to 
an  immense  mass  of  cortex,  and  internally  adds  slowly  to  the 
prismatic  layer.  This  zone  is  the  so-called  cambium.  Its  cells 
contain  deeply  staining  plasmic  contents  in  addition  to  starch 


228 


BOTANICAL  GAZETTE 


[APRIL 


(fig- 5)  -  Whether  or  not  the  dividing  cylinder  is  more  than 
one  cell-layer  in  thickness  I  could  not  determine. 

The  cortex  is  very  bulky  and  consists  throughout  of  iso- 
diametric  parenchymatous  cells  abundantly  filled  with  starch 
together  with  a  little  oil.  The  cells  undergo  no  divisions,  but 
as  they  are  forced  outward  by  the  activity  of  the  cambium 
they  increase  very  greatly  in  size  {figs.  6  and  7).  In  this  way 
by  the  growth  of  the  cells  in  all  directions  the  cortex  expands 
upwards  and  downwards  as  well  as  outwards,  and  as  a  result 
carries  up  the  older  leaves  to  a  height  considerably  above  the 
stem  apex.  This  is  illustrated  in  fig.  68,  in  which  the  unbroken 
lines  represent  the  form  which  the  cortex  would  take  if  its  cells 
underwent  no  enlargement  as  they  are  pushed  out  from  the 
cambium,  and  the  dotted  lines  the  form  and  dimensions  which  it 
actually  assumes. 

The  stem  apex  lies  at  the  base  of  the  conical  depression 
formed  in  the  manner  just  explained.  In  small  plants  it  is  dis¬ 
tinguishable  in  longitudinal  sections  as  a  slight  elevation  {fig.j)', 
in  older  plants  it  is  merely  a  flattened  area  between  the  bases  of 
the  young  leaves  {fig.  4). 

The  method  of  growth  of  the  apical  meristem  was  first  cor¬ 
rectly  described  by  Hegelmaier  (2).  Hofmeister  (1)  had  erro¬ 
neously  ascribed  it  to  the  segmentation  of  an  apical  cell,  having 
been  led  to  that  conclusion  probably  by  too  exclusive  study  of 
young  plants.  There  is  neither  an  apical  cell  nor  such  a  group 
of  initials  as  might  result  from  the  division  of  a  rectangular  apical 
cell  like  that  of  the  Marattiaceae.  Only  two  or  three  layers  of  cells 
show  their  meristematic  nature  by  their  contents.  The  superficial 
layer  appears  to  divide  only  in  an  anticlinal  direction  except  when 
young  leaves  are  about  to  be  formed  ;  but  this  layer,  as  Hegelmaier 
showed,  can  on  no  account  be  regarded  as  a  dermatogen. 

Although  all  the  species  of  Isoetes  are  perennial,  only  a 
small  portion  of  the  plant  persists  from  year  to  year.  The  roots, 
the  leaves,  and  the  bulky  cortex  are  shed  or  decay  annually, 
and  are  as  often  renewed  from  the  stem  apex  and  the  meristem¬ 
atic  zone  which  surrounds  the  small  central  permanent  cylinder. 


1900] 


SPOROPHYLLS  AND  SPORANGIA  OF  ISOETES 


229 


THE  LEAF  AND  LIGULE. 

In  its  earliest  recognizable  form  the  leaf  rudiment  seen  from 
above  is  a  crescent-shaped  band  of  meristematic  cells,  curved 
about  the  stem  apex.  Sections  show  that  it  arises  from  the 
superficial  cells  of  the  stem  apex,  and  is  soon  pushed  up  into  a 
low  broad  mass,  highest  in  the  middle  and  inclined  inwards. 
The  ligule  appears  very  early,  and  the  leaf  becomes  distinguish¬ 
able  into  a  proximal  part  somewhat  triangular  in  section  and 
destined  to  bear  the  sporangium,  and  a  distal  part  approximately 
circular  in  section  and  destined  to  become  the  chlorophyllous 
region.  In  correlation  with  the  rapid  development  of  the  spo¬ 
rangium,  the  growth  of  the  leaf  is  at  first  almost  confined  to  the 
basal  region.  Compare,  e.  g.,  the  three  leaves  shown  in  fig.  8  ; 
tranverse  sections  would  show  the  rapid  growth  of  the  basal 
region  in  a  still  greater  degree.  This  region  continues  to  widen 
as  the  leaf  is  pushed  outward,  by  the  formation  of  new  leaves 
and  the  diametral  enlargement  of  the  stem  ;  but  longitudinally, 
except  for  a  slight  addition  below  the  sporangium,  there  is 
only  sufficient  growth  to  accommodate  the  sporangium,  velum, 
and  ligule. 

When  the  sporangium  is  well  under  way  the  region  of  rapid 
multiplication  and  growth  of  cells  is  transferred  to  the  part  above 
the  ligule.  The  cells  here  are  arranged  with  beautiful  regular¬ 
ity,  and  growth  is  so  rapid  that  this  soon  becomes  the  most 
prominent  part  of  the  leaf.  The  maximum  diameter,  so  far  as 
the  number  of  cells  is  concerned,  is  speedily  attained,  and 
growth  thereafter  is  only  in  the  longitudinal  direction.  At  first 
every  part  of  the  leaf  rudiment  is  meristematic,  but  in  a  short 
time  the  apex  passes  over  into  permanent  tissue.  This  change 
into  permanent  tissue  progresses  gradually  downward  until 
finally  the  whole  leaf  is  involved.  For  some  time  a  region  of 
ever  narrowing  extent  above  the  ligule  continues  in  active 
division,  but  there  is  present  no  sharply  marked  or  persistent 
meristematic  zone,  as  seems  to  be  implied  in  Farmer’s  account. 
The  leaf  is  still  quite  small  when  all  cell  divisions  have  practi¬ 
cally  ceased,  and  its  further  elongation,  which  may  amount  to 


230 


BOTANICAL  GAZETTE 


[APRIL 


400  or  500  per  cent.,  is  accomplished  by  th*e  growth  of  the  indi¬ 
vidual  cells. 

The  formation  of  the  air  cavities  is  interesting,  since  it  is 
comparable  in  some  respects  to  the  differentiation  of  trabecular 
and  sporogenous  tissue  in  the  sporangium.  In  a  leaf  such  as  is 
represented  in  cross  section  in  fig.  9,  there  is  yet  no  sign  of  the 
air  chambers.  Increase  of  diameter  is  actively  going  on  and 
the  whole  leaf  is  still  meristematic.  In  the  leaf  shown  in  fig. 
10  the  position  of  the  future  air  chambers  is  indicated  by  four 
symmetrically  placed  groups  of  cells  which  have  lost  most  of 
their  contents.  The  peripheral  cells  of  the  leaf,  the  central 
cells,  and  four  radiating  bands  which  appear  in  cross  section  as 
spokes  arranged  in  the  form  of  the  sign  -\-  continue  to  grow  and 
are  distinguishable  by  their  larger  more  densely  filled  cells. 
Stained  with  Delafield’s  haematoxylin  and  erythrosin  these 
cells  show  deep  red  cytoplasmic  contents  and  large  nuclei  in 
which  the  red  staining  predominates  ;  while  in  the  areas  which 
are  to  become  air  chambers  the  cytoplasmic  contents  have 
almost  entirely  disappeared,  but  the  nuclei  still  retaining  their 
chromatin  stain  intensely  with  haematoxylin.  When  only  a 
nuclear  stain  is  employed,  such  as  iron-alum-haematoxylin,  the 
four  non-protoplasmic  areas  are  rendered  very  prominent  by 
their  black  nuclei.  Longitudinal  sections  show  that  the  regions 
which  are  thus  sharply  distinct  in  cross  section  run  lengthwise  of 
the  leaf  in  unbroken  bands  from  just  above  the  ligule  nearly  to 
the  apex,  and  there  are  as  yet  no  air  cavities. 

The  air  chambers  are  formed  lysigenously.  The  growing 
tissues  generate  a  tension  in  the  empty  cells,  and  as  a  result 
these  are  ruptured  irregularly,  and  small  cavities  appear,  sepa¬ 
rated  by  diaphragms  or  plates  of  cells  extending  across  from 
the  central  to  the  peripheral  growing  regions.  As  the  leaf  elon¬ 
gates,  the  air  cavities  increase  in  size,  while  the  diaphragms  drawn 
farther  and  farther  apart  lose  their  protoplasm  to  the  surrounding 
cells.  When  once  this  splitting  into  diaphragms  and  cavities 
has  occurred,  it  is  not  repeated  ;  there  remains  no  meristem  in 
which  they  may  be  generated.  Occasionally  single  diaphragms 


1900 J  SPOROPHYLLS  AND  SPORANGIA  OF  ISOETES  231 

of  unusual  thickness  may  be  again  ruptured,  but  no  considerable 
increase  in  their  number  ever  occurs  after  their  first  formation. 

It  is  easy  with  the  low  power  of  the  microscope  to  count  the 
diaphragms  in  leaves  floating  upon  a  little  water  on  a  slide. 
The  usual  number  is  from  fifty  to  seventy,  and  is  quite  as  many 
in  young  leaves  three  fourths  of  an  inch  long  as  in  leaves  fully 
formed.  It  is  instructive,  too,  as  proving  the  absence  of  a  defi¬ 
nite  meristematic  zone,  to  count  the  average  number  of  super¬ 
ficial  cells  which  intervene  between  the  diaphragms.  In  very 
young  leaves  this  is  from  three  to  six  or  eight  throughout  the 
whole  length,  but  in  older  leaves  it  is  much  greater,  varying 
from  twelve  to  twenty  in  the  tip  region  to  forty  to  sixty  in  the 
middle  and  basal  regions,  which  remain  longest  in  the  meristem¬ 
atic  condition. 

The  diaphragms,  I  think,  are  quite  functionless,  and  their 
existence  merely  incidental  to  the  manner  of  origin  of  the  air 
chambers.  They  are  too  delicate  to  serve  for  mechanical  sup¬ 
port,  which  is  sufficiently  secured  by  the  four  longitudinal  bands 
already  described.  The  position  of  the  air  chambers  and  longi¬ 
tudinal  bands  between  them  in  relation  to  the  axis  of  the  plant 
is  always  the  same  as  that  indicated  in  figs.  10,  //,  44.  Near 
the  ligule  the  air  spaces  are  less  regular,  and  instead  of  four 
of  them  symmetrically  placed  we  find  many  irregular  ones. 
Behind  the  sporangium  the  dorsal  longitudinal  band  of  living 
cells,  and  sometimes  the  two  lateral  ones,  are  well  marked,  but 
there  are  no  large  distinct  air  spaces.  The  vascular  bundle  of 
the  leaf  is  as  characteristic  as  that  of  the  stem.  My  observations, 
referring  chiefly  to  the  changes  of  form  of  the  bundle,  were  made 
with  the  view  of  discovering  whether  there  is  any  definite  relation 
between  it  and  the  sporangium  or  the  ligule,  and  whether  it 
presents  any  evidence  that  the  leaf  of  Isoetes  has  been  reduced 
from  a  more  complex  type.  The  leaf  trace  can  first  be  recognized 
inthebaseof  the  young  leaf  and  in  thestem  region  below  it  towards 
the  central  bundle.  The  xylem  elements  are  first  differentiated,, 
and  consist  of  five  or  six  tracheids  grouped  into  a  cylinder  and 
surrounded  by  a  sheath  of  parenchymatous  cells  with  dense 


232 


BOTANICAL  GAZETTE 


[APRIL 


contents.  These  parts  can  be  traced  later  to  the  corresponding 
parts  of  the  axial  bundle.  Behind  the  sporangium  the  xylem 
spreads  out  into  a  broad  band  in  which  the  amount  of  xylem 
parenchyma  is  greatly  increased,  and  the  tracheids  are. in  five  or 
six  scattered  groups.  Above  the  sporangium  the  xylem  con¬ 
tracts  again  into  a  cylinder,  and  lies  between  the  cornua  of  the 
ligule  base.  A  more  striking  change  occurs  above  the  ligule 
where  the  xylem  elements  suffer  an  extreme  diminution,  there 
being  in  that  region  in  I.  echinospora  only  a  single  imperfect 
central  tracheid  surrounded  by  a  sheath  of  parenchyma  {Jigs,  p, 
10).  Occasionally  in  /.  echinospora ,  and  usually  in  I.  Englemanni, 
two,  sometimes  three,  other  such  groups  can  be  traced  up  the 
leaf. 

The  phloem  is  best  represented  in  the  chlorophyll-bearing 
portion  of  the  leaf.  It  there  consists  of  two  strap-shaped  bands 
on  the  dorsal  side,  more  or  less  united  by  their  edges,  so  as  partly 
to  surround  the  xylem.  In  less  distinct  form  the  phloem  may 
be  traced  downwards  to  the  region  of  the  central  bundle. 

The  development  of  the  ligule  was  accurately  described  by 
(Hofmeister  i),  and  also  by  Hegelmaier  (] ).  The  latter  refers 
its  origin  to  more  than  one  cell.  Since  the  former  gives  few 
figures,  however,  and  the  latter  none,  I  shall  again  briefly 
outline  the  course  of  growth  and  illustrate  it  with  a  few 
drawings.  The  ligule  originates  from  a  single  large  vesicular 
cell  protruding  from  the  ventral  face  of  the  leaf  rudiment.  Pro¬ 
vision  for  its  rapid  growth  is  shown  in  the  large  size  of  the  nucleus 
of  this  cell,  and  the  density  of  the  cytoplasm  {jigs.  12 ,  ij).  The 
first  division  is  always  parallel  to  the  face  of  the  leaf  {Jigs.  14, 
ij) ,  and  usually  the  second  division  is  parallel  to  the  first. 
The  ligule  of  I.  lacustris  is  described  as  passing  through  a  fila¬ 
mentous  stage  ;  but  in  I.  echmospora  and  I.  Engelma7ini  it  is 
hardly  worth  while  to  distinguish  such  a  stage,  for  the  filament 
never  consists  of  more  than  three  cells.  The  terminal  cell  then 
divides  in  a  vertical  plane  at  right  angles  to  the  first  wall  (  figs. 
16 ,  18).  Other  vertical  divisions  follow  until  the  ligule  has 
become  a  plate  of  cells  of  very  regular  arrangement.  Figs.  18 


1900] 


SPOROPHYLLS  AND  SPORANGIA  OF  ISOETES 


233 


and  ig  are  median  sections  of  the  ligule  made  tangentially  to 
the  face  of  the  leaf.  Longitudinal  sections  are  shown  in  figs.  26 , 
2j,  28 ,  55,  55.  Growth  in  length  and  breadth  continues  very 
rapid,  and  the  ligule  soon  overtops  the  leaf  (fig.  8).  For 
some  time  it  remains  a  single  layer  of  cells  in  thickness,  but 
eventually  it  becomes  double  throughout  most  of  its  extent. 
The  doubling  begins  in  the  middle  region  near  the  base  and 
extends  in  all  directions,  never  reaching  the  apex  or  margin 
however,  which  remain  to  the  last  but  one  layer  in  thickness 
(fig.  21).  The  expanded  part  soon  reaches  its  maximum 
growth.  Not  so  the  foot  region  ;  this  becomes  quite  massive 
and  deeply  embedded  in  the  tissue  of  the  leaf,  especially  at  the 
sides  which  grow  upward  and  downward  into  two  prominent 
cornua.  Figs.  22-25  may  help  to  explain  the  form  of  the  base  of 
the  ligule.  Fig.  25  is  a  transverse  section  of  the  leaf  cutting 
across  the  cornua  above  the  main  place  of  union  of  the  ligular 
and  leaf  tissues.  Sections  below  it  show  the  cornua  connected 
by  a  transverse  band  embedded  in  the  leaf  ;  and  sections  still 
lower  would  show  portions  of  the  cornua  only.  The  other 
figures  need  no  fuller  explanation  than  that  accompanying  the 
plates. 

Along  with  the  growth  of  the  ligule  there  comes  about  a 
differentiation  of  the  cells  composing  it.  There  may  be  said  to 
be  four  regions.  The  base  is  closely  surrounded  by  a  layer  of 
small  deeply-staining  gland-like  cells  (.?  in  figs.  22,58)  which 
we  may  call  the  sheath.  It  forms  a  conspicuous  layer,  every¬ 
where  investing  the  base  of  the  ligule,  and  becoming  continuous 
with  the  superficial  cells  of  the  leaf.  Next  to  the  sheath  is  an 
irregular  layer  or  band  of  large  empty  cells,  the  glossopodium 
(g  in  figs.  22 ,  58;  see  also  figs.  25-25).  The  glossopodium 
appears  to  form  the  base  of  the  ligule,  but  the  true  base  includes 
the  sheath  which,  as  a  study  of  the  development  shows,  is 
derived  from  the  lowermost  cell  of  the  young  ligule  (fig-  58). 
Above  the  glossopodium  are  smaller  cells  containing  protoplasm 
and  forming  the  greater  part  of  the  ligule.  The  apex  and 
margin  of  older  ligules  constitute  the  fourth  region  ;  the  cells  are 


234 


BOTANICAL  GAZETTE 


[APRIL 


shrunken  and  contorted,  their  nuclei  broken  down,  and  the  cyto¬ 
plasm  disorganized. 

A  study  of  the  ligule  of  Isoetes  to  be  complete  must  be 
accompanied  by  a  comparative  examination  of  the  ligule  of 
Selaginella.  With  this  in  view  I  have  studied  the  origin  and 
growth  of  the  ligule  in  .S.  Marte?isii  and  vS.  apus ,  and  compared 
my  sections  with  the  excellent  drawings  of  Professor  Harvey 
Gibson  (2).  Professor  Farmer  (1)  has  expressed  the  view  that 
the  ligules  of  Isoetes  and  Selaginella  have  little  in  common 
except  their  position  and  name.  I  have  been  led  to  quite  the 
contrary  conclusion,  to  hold  in  fact  that  there  is  a  very  close 
homology  between  the  two.  What  has  appealed  most  to  me, 
in  addition  to  the  position  of  the  organs,  is  the  similarity  of  the 
regions  of  which  both  are  seen  to  consist.  The  ligule  of  Selag¬ 
inella  has  a  glossopodium  of  large  empty  cells,  sheathed  by  a 
gland-like  layer,  and  shows  also  two  upper  regions,  one  of  living 
and  one  of  disorganizing  cells.  The  two  are  alike  also  in  the 
absence  of  chlorophyll,  starch,  and  intercellular  spaces ;  and 
both  show  their  embryonic  character  by  passing  their  maximum 
of  growth  before  the  leaf  has  reached  its  greatest  functional 
activity.  Differences  are  to  be  expected,  of  course,  and  are 
chiefly  these:  the  ligule  of  Isoetes  arises  from  a  single  cell,  that 
of  Selaginella  from  a  group  of  cells  ;  and,  whereas  the  ligule 
of  Isoetes  is  almost  from  the  beginning  a  conspicuous  part  of  the 
leaf,  that  of  Selaginella  is  rather  late  in  making  its  appearance, 
no  trace  of  it  being  discoverable  unt.il  after  the  sporangium  rudi¬ 
ment  is  plainly  perceptible. 

THE  SPORANGIUM. 

The  sporangium  has  repeatedly  been  made  the  object  of 
investigation  during  the  last  fifty  years.  Hofmeister  (1)  was 
the  first  to  make  a  careful  study  of  its  origin  and  development. 
Though  his  view  that  the  sporangium  can  be  traced  back  to  a 
single  cell  has  been  discredited  by  later  observers,  I  hope  to  show 
that  his  error  was  largely  due  to  his  exclusive  dependence  upon 
longitudinal  sections.  Except  for  his  failure  to  see  the  true 


1900] 


SPOROPHYLLS  AND  SPORANG/A  OP  ISOETES 


235 


nature  of  the  sporangium  rudiment  as  a  transverse  row  of  cells, 
his  account  is  surprisingly  accurate  when  the  imperfect  methods 
of  sectioning  and  staining  of  that  time  are  taken  into  considera¬ 
tion. 

According  to  Hegelmaier  (2)  and  Tschistiakoff  (1)  the 
sporogenous  tissue  is  differentiated  out  of  a  considerable  mass 
of  deep-lying  meristem  between  the  epidermis  and  the  vascular 
bundle. 

Goebel  (1)  agrees  substantially  with  the  two  preceding 
authors,  but  is  more  explicit  in  his  description.  The  Anlage  of 
the  sporangium  according  to  him  is  a  group  of  cells  of  the  leaf 
base,  chiefly  the  three  upper  layers.  The  outer  layer  gives  rise 
to  the  sporangium  wall,  and  the  hypodermal  layer  to  the  arche- 
sporium  from  which  all  the  spore  mother  cells,  trabeculae,  and 
tapetum  are  derived.  Goebel’s  account,  as  confirmed  and 
restated  by  Sadebeck  (1)  in  Schenck’s  Ha?idbuch  der  Botanik,  has 
formed  the  basis  of  all  the  text-book  descriptions  of  the  sporan¬ 
gium  of  Isoetes  written  since  that  time. 

The  latest  student  in  this  field  is  Bower  (5),  whose  descrip¬ 
tion  is  confirmatory  of  Goebel’s  except  that  he  traces  the  origin 
of  the  sporangium  to  a  group  of  superficial  cells.  This  differ¬ 
ence,  however,  is  of  the  very  greatest  importance.  For  whereas 
the  derivation  of  the  archesporium  by  periclinal  divisions  of 
superficial  cells  is  the  rule  in  Pteridophytes,  the  origin  of  the 
sporogenous  tissue  from  a  hypodermal  layer  separated  from  the 
beginning  from  the  epidermis  is  a  spermatophyte  character.  The 
result  of  Bower’s  work  then  is  to  put  Isoetes  in  line  with  other 
Pteridophytes  in  respect  to  the  origin  of  the  archesporium. 

My  own  results  are  in  the  main  confirmatory  of  Bower’s  as 
to  the  origin  of  the  sporangium,  though  with  variations  in  minor 
details  which  may  be  due  to  specific  differences  (Bower  studied 
/.  lacustris ) ;  but  as  to  the  later  stages  of  development,  especially 
of  the  megasporangium,  I  cannot  make  my  observations  harmo¬ 
nize  with  any  accounts  hitherto  written. 

It  will,  of  course,  be  apparent,  when  so  many  discrepancies 
appear  in  the  descriptions  of  different  investigators,  that  the 


236 


BOTANICAL  GAZETTE 


[APRIL 


study  must  be  one  which  involves  considerable  technical  diffi¬ 
culty.  This  is  attributable  ( 1 )  to  the  absence  of  an  elongated 
axis  and  internodes  and  the  consequent  crowding  of  the  sporo- 
phylls,  and  (2)  to  the  early  appearance  of  the  sporangium  and 
the  consequent  difficulty  of  distinguishing  it  from  the  other 
meristematic  tissues  in  which  it  is  placed.  The  kinds  of  evidence 
on  which  I  have  relied  in  my  interpretations  may  be  stated  briefly 
as  follows  : 

1.  Study  was  made  of  sporangia  whose  sporogenous  tissue 
was  already  distinct  and  unmistakable.  Then  by  comparisons 
with  successively  younger  sporophylls  the  attempt  was  made  to 
trace  the  sporangium  to  its  earliest  rudiment. 

2.  A  careful  comparison  was  made  of  sections  in  the  three 
planes,  longitudinal,  transverse,  and  tangential.  This  involved 
the  waste  of  a  great  deal  of  material.  For  it  will  be  made  clear 
by  a  glance  at  fig.  4  that  sections  made  longitudinal  to  the  stem 
could  give  longitudinal  sections  of  very  few  young  leaves,  and 
oftener  than  not  would  fail  in  this  altogether,  since  the  leaves 
have  a  spiral  arrangement  ;  while,  in  order  to  obtain  transverse 
and  tangential  sections,  one  must  cut  obliquely  to  the  stem  with¬ 
out  possessing  any  clue  by  which  to  determine  the  proper  angle 
of  obliquity. 

3.  The  position  of  the  vascular  bundle  enables  one  to 
determine  whether  the  sections  are  truly  longitudinal,  and  which 
of  a  number  of  serial  longitudinal  sections  is  exactly  median. 
This  help  is  available  only  after  the  sporangium  is  distinctly  out¬ 
lined,  and  somewhat  advanced  in  development,  for  in  case  of 
very  early  stages  of  the  sporangium,  the  vascular  bundle  has  not 
yet  been  differentiated. 

4.  In  such  early  stages  one  must  depend  very  largely  upon 
the  ligule,  which  in  position  and  outline  is  so  definite,  and  in 
manner  of  growth  so  regular  as  to  make  it  of  the  highest  impor¬ 
tance  in  assisting  one  to  orient  the  sections. 

5.  The  sporogenous  tissue  is  often  distinguishable  from 
vegetative  tissue  by  a  difference  in  staining.  There  are  three 
periods  when  this  difference  is  most  manifest.  The  superficial 


igoo]  SPOROPHYLLS  AND  SPORANGIA  OF  ISOETES  237 

cells  which  form  the  earliest  rudiment  of  the  sporangium  fre¬ 
quently  take  a  distinctive  cytoplasmic  staining,  especially  in 
material  fixed  in  Flemming’s  solution.  It  must  be  confessed 
that  this  means  of  recognizing  sporogenous  tissue  is  not  so  trust¬ 
worthy  as  one  could  wish,  for  at  this  period  of  the  leaf’s  history 
all  the  tissues  are  meristematic,  and  hence  readily  susceptible  to 
protoplasmic  stains.  One  who  studies  the  origin  of  sporangia 
in  Lycopodium  or  Selaginella  meets  with  the  same  difficulty  in 
those  plants,  a  difficulty  in  my  experience  quite  as  great  in  these 
cases  as  in  Isoetes.  When  the  superficial  layer  of  the  sporan¬ 
gium  has  assumed  its  character  as  an  epidermis,  the  deeper  lying 
sporogenous  cells  are  easily  distinguishable  by  stain  reactions 
from  the  surrounding  tissues.  At  a  later  period  the  spore 
mother  cells  selected  out  of  the  general  internal  mass  of  the 
sporangium  become  quite  distinct  on  account  of  their  denser  con¬ 
tents  and  more  intense  staining. 

Longitudinal  sections  of  young  leaves  show  no  space  between 
the  base  of  the  ligule  and  the  stem.  At  this  time  there  is  still 
an  active  uplifting  of  cells  above  the  general  stem  level,  a  con¬ 
tinuance  of  the  process  by  which  the  leaf  first  emerged.  When 
the  ligule  has  grown  sufficiently  to  contain  eight  or  ten  cells  in 
longitudinal  section  the  space  below  it  is  occupied  by  one  large 
cell  with  dense  cytoplasmic  contents  ( fig .  26) .  The  next  change 
which  takes  place  is  a  transverse  division  of  this  cell  as  shown 
in  figs.  27,  28.  Comparisons  of  successive  serial  sections 
show  that  the  two  cells  shaded  in  fig.  27  form  the  middle  of  a 
group  of  cells  arranged  transversely  to  the  leaf.  This  group  of 
cells,  distinguishable  in  good  preparations  by  their  deeper  stain¬ 
ing  and  larger  nuclei,  constitute  the  rudiment  of  the  sporangium. 
In  order  to  learn  its  extent  and  arrangement  recourse  must  be 
had  to  transverse  and  tangential  sections. 

Most  transverse  sections  of  this  early  stage  of  the  sporangium 
show  that  it  is  five  cells  in  width.  Whether  or  not  these  can  be 
traced  back  to  a  still  smaller  number  I  am  in  doubt.  Fig.  2(7 
certainly  shows  an  example  where  the  transverse  row  consists  of 
only  three  cells,  and  it  is  clear  that  the  shaded  cells  of  fig.  jo 


238 


BOTANICAL  GAZETTE 


[APRIL 


may  have  had  their  origin  in  three  similar  to  those  of  fig.  2g . 
But  I  have  succeeded  in  getting  only  two  such  cases  as  that  of 
fig.  2Q ,  one  in  I.  echinospora  and  one  in  I.  Engelmamii ,  and  have 
failed  altogether  to  obtain  a  tangential  view. 

Tangential  sections  of  the  leaf  at  this  early  stage  are  almost 
uninterpretable.  The  face  of  the  leaf  is  so  closely  pressed 
against  the  back  of  the  next  younger  one  that  it  is  quite  impos¬ 
sible  in  most  instances  to  distinguish  the  tissues  of  the  two  leaves 
or  to  determine  what  is  a  truly  tangential  section.  That  shown 
in  fig.  31  was  such  as  to  admit  of  certain  interpretation.  The 
shaded  cells  occupy  the  surface  of  the  leaf  and  clearly  corre¬ 
spond  to  the  group  which  we  have  already  examined  in  longi¬ 
tudinal  and  transverse  sections.  It  is  probable  that  another  cell 
seen  in  the  adjacent  section  to  the  left  of  those  figured  belongs 
to  the  same  group,  making  the  total  number  of  cells  seven. 

It  is  evident  from  a  comparison  of  my  figs.  26-28  with  figs. 
104-106  of  Professor  Bower’s  plates,  that  the  longitudinal 
growth  of  the  leaf  base  of  I.  lacustris  is  much  more  rapid  than 
that  of  I.  echinospora ;  and  his  figures  though  not  his  text  sug¬ 
gest  that  the  six  superficial  cells  which  make  up  the  sporangium 
Anlage  are  derived  from  not  more  than  three  rows  and  probably 
from  but  two.  If  this  suggestion  be  correct,  it  would  bring 
Bower’s  and  Hofmeister’s  accounts,  so  far  as  regards  longitudinal 
sections,  into  harmony  with  each  other,  and  with  the  foregoing 
account  of  I.  echhiospora. 

The  young  sporangium,  situated  as  it  is  on  the  hollow  side  of 
the  leaf  crescent,  projects  little  if  at  all  from  the  surface.  By  its 
rapid  growth,  however,  it  soon  forms  an  oval  prominence  at  first 
wider  than  long,  then  nearly  circular  in  surface  view,  and  finally 
considerably  longer  than  wide.  In  its  development  I  have  not 
been  able  to  establish  any  regular  order  of  sequence.  Starting 
from  such  a  beginning  as  figured  in  fig.  26 ,  it  is  certain  that 
transverse  and  longitudinal  divisions  are  the  first  to  occur. 
Then  periclinal  walls  appear  ( fig .  36) .  The  middle  cells  of  the 
sporangium  rudiment  are  at  first  most  active  in  dividing,  not 
only  in  respect  to  surface  growth,  but  in  periclinal  divisions  also. 


1900] 


SPOROPHYLLS  AND  SPORANGIA  OF  ISOETES 


239 


Sections  adjacent  to  that  represented  in  fig.  31  show  three  or 
four  hypodermal  cells  which  have  been  cut  off  from  the  middle 
cells  of  the  group  and  evidently  belong  to  the  same  series. 

There  is  at  no  time  a  single  complete  hypodermal  layer 
which  may  properly  be  termed  an  archesporium.  For  when  the 
middle  cells  have  just  completed  their  periclinal  divisions  the 
lateral  cells  are  still  undivided,  and  by  the  time  the  lateral  cells 
have  undergone  their  first  periclinal  division  the  middle  of  the 
sporangium  is  at  least  three  layers  deep.  A  very  good  example 
of  this  is  seen  in  fig.  42,  which  represents  the  side  of  quite  a 
large  sporangium. 

The  growth  of  the  sporangium  is  carried  on  most  actively  by 
the  two  or  three  outer  layers  of  cells,  as  is  evidenced  by  their 
large  size  and  deeper  staining,  and  the  frequency  with  which 
they  are  found  in  karyokinesis.  The  divisions  of  the  superficial 
layer  are  by  no  means  limited  to  those  in  anticlinal  planes,  as 
is  usually  the  case  with  the  external  cells  of  sporangia,  but  for 
a  long  time  they  continue  to  add  to  the  inner  mass  by  periclinal 
divisions.  In  the  sporangium  of  which  fig.  39  shows  a  section, 
as  many  as  eight  or  ten  of  the  external  cells  were  in  the  act  of 
periclinal  division.  Even  in  so  old  a  sporangium  as  that  shown 
in  fig.  43  the  same  process  is  still  in  continuance.  The  cells 
marked  with  a  cross  have  evidently  been  derived  from  the 
external  layer.  Though  in  older  sporangia  the  additions  so 
made  go  to  form  part  of  the  sporangium  wall,  there  can  be  no 
question  that  in  the  younger  sporangia  they  add  to  the  true 
sporogenous  tissue.  The  bearing  of  this  fact  upon  the  question 
of  what  constitutes  an  archesporium  will  be  considered  further 
on. 

It  seems  necessary  to  digress  at  this  point  in  order  to  make 
clear  some  features  in  which  the  preceding  account  differs 
from  what  has  been  recorded  by  previous  observers.  Both 
Hegelmaier  (2)  and  Tschistiokoff  (1)  assert  that  the  wall  of  the 
sporangium  is  from  the  beginning  (“von  Anfang  an  gesondert”) 
separated  from  the  inner  complex,  and  emphasize  with  great 
distinctness  the  deep-*seated  origin  of  the  sporogenous  tissue. 


240 


BOTANICAL  GAZETTE 


[APRIL. 


Goebel  (i)  states  his  approval  of  Hegelmaier’s  view,  but  the 
occasional  periclinal  division  of  the  external  cells  does  not 
escape  his  notice,  though  he  considers  it  as  merely  adding  to 
the  thickness  of  the  wall.  Bower  (5),  on  the  other  hand, 
observed  both  the  superficial  origin  of  the  sporangium  and  the 
failure  of  the  first  periclinal  divisions  to  completely  delimit  the 
archesporium. 

As  already  stated,  I  do  not  find  the  outer  wall  separate 
from  the  sporogenous  complex  from  the  beginning.  On  the 
contrary,  it  is  distinctly  active  in  increasing  the  dimensions, 
of  the  sporangium.  Ultimately  the  superficial  layer  loses 
some  of  its  protoplasmic  contents,  and  assumes  the  appearance 
of  an  epidermis.  It  sometimes  happens  that  this  separation  of  a 
wall  layer  occurs  quite  early  {fig.  41),  but  oftener  it  is  not  till 
the  sporangium  has  come  to  consist  of  many  hundred  cells. 
Even  then  periclinal  divisions  do  not  entirely  cease. 

According  to  my  observations  there  is  no  regularity  in  the 
arrangement  of  the  cells  within  the  sporangium.  The  discovery 
of  this  was  a  great  surprise  to  me,  for  Goebel’s  statement  is  very 
explicit:  “Each  of  the  cells  composing  the  archesporium  has 
an  independent  growth,”  and  in  this  he  has  been  corroborated 
by  Sadebeck  and  Farmer.  Bower  has  not  traced  the  history 
of  the  sporangium  with  any  fullness ;  he  merely  states  that  his 
results  are  confirmatory  of  Goebel’s  and  his  figures  certainly 
convey  the  impression  that  each  cell  of  the  archesporium  has  an 
independent  growth.  But  he  has  made  use  of  the  same  style  of 
drawing  in  representing  the  sporangia  of  other  genera  (Lycopo¬ 
dium,  Selaginella,  Equisetum),  in  which  no  such  claim  is  made. 
In  view  of  my  own  observations  and  of  Bower’s  drawings,  it  is 
difficult  to  know  just  how  much  is  meant  by  the  phrase  “inde¬ 
pendent  growth.” 

In  the  case  of  bryophyte  antheridia  the  primary  spermatog- 
enous  cells  are  clearly  distinguishable  throughout  the  whole 
development  of  the  antheridium,  although  each  may  become 
divided  up  into  a  hundred  or  more  sperm  mother  cells.  The 
individuality  of  the  original  cells  is  marked  in  several  ways : 


igoo]  SPOROPHYLLS  AND  SPORANGIA  OF  ISOETES  241 

their  outer  walls  remain  straight  and  become  thicker  than  those 
which  subsequently  appear  within  them ;  and  the  incomplete 
separation  of  the  derivatives  of  any  single  primary  sperm  cell 
from  one  another  and  their  complete  separation  from  those  of 
other  primary  cells  are  shown  by  their  dividing  concurrently. 
I  have  frequently  observed  in  the  antheridia  of  Polytrichum, 
Porella,  Marchantia,  and  Asterella  that  all  the  cells  derived 
from  one  of  the  primary  sperm  cells  enter  into  karyokinesis 
together,  finish  their  division,  and  enter  into  the  resting  condi¬ 
tion  together,  quite  independently  of  what  may  be  going  on  in 
the  derivatives  of  other  primary  cells.  In  such  cases  it  is  quite 
proper  to  speak  of  an  independent  growth  ;  for  the  separation 
and  isolation  of  each  group  by  thickened  walls  are  sufficient  to 
insure  a  simultaneous  exposure  and  obedience  of  all  the  cells  to 
the  physiological  stimulus  which  induces  karyokinesis. 

Are  there  any  indications  of  such  independent  growth  in  the 
sporangium  of  Isoetes  ?  I  can  find  none,  either  in  the  arrange¬ 
ment  of  the  tissues  or  in  the  presence  of  thickened  walls  which 
mark  the  boundaries  of  the  original  archesporial  cells,  or  in  the 
simultaneous  entrance  of  the  cells  of  each  group  into  the  phases 
of  division.  All  the  mature  cell  walls  of  a  growing  sporangium 
are  of  equal  thickness;  and  in  marked  contrast  to  what  is  seen 
in  the  leaves  there  is  no  regularity  of  stratification  or  lining-up 
of  the  cells.  I  am  forced  to  conclude  that  the  sporangium 
of  Isoetes  (at  least  of  I.  echinospora  and  I.  Enge Imamu) ,  just  as 
the  microsporangium  of  angiosperms,  grows  as  a  unit  and  not  as 
a  number  of  individual  segments. 

Before  continuing  the  subject  of  the  development  of  the 
sporangium  it  will  be  convenient  to  consider  the  formation  of 
the  velum.  The  velum  makes  its  appearance  very  early  in  the 
history  of  the  sporangium,  almost  as  soon  in  fact  as  the  first 
periclinal  divisions  of  the  superficial  cells.  It  is  formed  immedi¬ 
ately  below  the  ligule.  Hofmeister  (1)  says:  “Of  the  two  cells 
into  which  by  a  transverse  septum  the  cell  underneath  the  place 
of  insertion  of  the  ligule  is  divided  the  upper  one  becomes  the 
primary  cell  of  the  velum  and  the  lower  the  primary  mother  cell 


242 


BOTANICAL  GAZETTE 


[APRIL 


of  the  sporangium.”  It  has  already  been  said  that  Hofmeister 
was  in  error  because  of  failure  to  notice  the  lateral  extension  of 
the  sporangium  rudiment.  Even  allowing  for  this,  however,  I 
am  not  able  to  agree  fully  with  his  account.  It  appears  rather 
that  the  upper  tier  of  cells  while  giving  rise  to  the  velum  makes 
some  additions  at  the  same  time  to  the  sporangium.  In  other 
words,  the  velum  is  a  sterilized  portion  of  the  sporangium.  Some 
sections  seem  to  admit  of  this  interpretation  only,  though  others, 
such  as  fig.  32 ,  are  not  unfavorable  to  the  view  that  the  separa¬ 
tion  of  velum  and  sporangium  proper  is  accomplished  by  the 
first  transverse  division  of  the  sporangium  rudiment. 

Early  stages  of  the  velum  may  be  seen  in  figs.  32-36 ;  it  is 
at  this  time  a  transverse  row  of  slightly  projecting  cells.  Its 
cells  soon  become  comparatively  empty,  contrasting  strongly 
with  the  young  sporangium.  Growth  is  very  rapid  and  in  an 
upward  oblique  direction ;  in  some  cases  there  is  a  tendency  to  a 
downward  growth  also,  such  as  obtains  among  terrestrial 
species. 

The  velum  reaches  its  full  size  much  sooner  than  the  spo¬ 
rangium,  and  is  not  affected  by  the  changes  which  determine  the 
character  of  the  latter.  The  cells  of  the  interior  become  large 
and  lose  their  contents;  those  of  the  inner  surface  layer  —  that 
adjacent  to  the  sporangium  —  are  smaller  and  more  regular  in 
size  and  outline,  and  have-  a  semi-glandular  appearance.  In 
many  species  of  Isoetes  many  of  the  cell  walls  of  the  velum  and 
of  the  leaf  region  adjoining  the  ligule  become  lignified  and  take 
on  spiral  and  annular  thickenings.  I.  eclimospora  and  I.  Engel- 
mamii  offer  no  exception  in  this  respect,  the  thickenings  being 
much  more  pronounced  in  the  latter  species.  The  change  first 
appears  in  proximity  to  the  ligule,  and  spreads  thence  into  the 
remoter  parts  of  the  velum  and  of  the  leaf.  The  thickened  cells 
never  have  any  connection  with  the  vascular  bundle  {figs.  23 ,  24). 

FURTHER  DEVELOPMENT  OF  THE  MICROSPORANGIUM. 

In  origin  the  two  kinds  of  sporangia  are  identical,  and  for 
a  considerable  period  of  their  development  they  exhibit  no 


1 9oo]  SPOROPHYLLS  AND  SPORANGIA  OF  ISOETES  243 

observable  difference.  The  general  statement  of  the  text-books, 
following  Goebel  and  Sadebeck,  is  that  they  follow  the  same 
course  of  development  only  so  far  as  the  formation  of  the 
archesporium,  and  thereafter  may  be  distinguished  by  their 
manner  of  growth.  It  is  said  that  in  the  megasporangium  certain 
archesporial  cells  divide  only  by  periclinal  walls,  but  in  the 
microsporangium  all  the  archesporial  cells  divide  both  anticli- 
nally  and  periclinally,  and  that  in  this  respect  the  two  are  dis¬ 
tinguishable  from  the  archesporial  stage  on.  Such  is  not  the 
case  in  the  forms  which  I  have  studied.  In  these  all  the  arche¬ 
sporial  cells,  whether  of  megasporangium  or  microsporangium, 
undergo  divisions  in  all  directions,  and  the  similarity  of  the  two 
kinds  of  sporangia  continues  much  beyond  the  archesporial 
stage.  Not  only  do  they  agree  in  origin,  but  up  to  a  time  when 
they  are  eight  or  ten  cells  deep,  they  agree  absolutely  in  manner 
of  growth,  and  exhibit  no  histological  features  by  which  one  may 
determine  whether  a  given  sporangium  will  bear  microspores  or 
megaspores. 

As  an  example,  consider  the  sporangium  of  which  fig.  43 
represents  a  section.  It  had  advanced  so  far  beyond  the  arche¬ 
sporial  stage  as  to  contain  about  8000  cells.  From  the  position 
of  its  sporophyll  we  may  infer  it  was  destined  to  become  a 
megasporangium.  But  there  is  nothing  in  the  arrangement  or 
character  of  the  cells  or  in  their  mode  of  division  to  warrant 
that  prediction,  or  to  enable  us  to  say  such  a  group  of  cells  will 
become  a  trabecula,  and  such  a  group  will  produce  spores.  It 
has  the  characters  neither  of  a  megasporangium  nor  of  a  micro¬ 
sporangium,  but  is  as  yet  quite  undifferentiated. 

The  first  changes  which  occur  to  mark  the  microsporangium 
are  those  which  lead  to  the  differentiation  of  the  spore  mother 
cells  from  the  trabeculae,  sporangium  wall,  and  tapetum.  Pre¬ 
viously  there  has  been  no  essential  difference  in  the  cells  as  to 
size,  form,  or  contents,  excepting  the  external  layer.  But  when 
the  sporangium  is  approaching  a  limit  of  cell  multiplication,  that 
is,  when  the  number  of  cells  is  15,000-20,000,  certain  regions 
begin  to  lose  their  power  of  division  and  reaction  to  stains,  while 


244 


BOTANICAL  GAZETTE 


[APRIL 


other  regions  become  more  active  in  division  and  more  deeply 
stainable.  The  former  may  be  called  the  sterile  regions,  since 
they  form  the  walls,  trabeculae,  and  tapetum,  and  the  latter  the 
fertile  region,  since  they  give  rise  to  the  spores.  Even  in 
unstained  sections  the  difference  is  noticeable  as  one  of  relative 
abundance  of  protoplasmic  contents. 

At  first  it  is  difficult  to  see  clearly  the  limits  of  the  regions 
or  to  make  out  their  arrangement.  But  in  older  sporangia  they 
are  seen  to  be  disposed  in  irregular  bands  extending  from  the 
base  of  the  sporangium  outwards  to  the  wall.  The  published 
drawings,  and  unfortunately  in  some  cases  the  written  descrip¬ 
tion  also,  are  calculated  to  convey  an  erroneous  idea  of  the 
trabeculae.  They  are  not  partitions,  but,  though  irregular  in 
outline  and  frequently  branched  and  anastomosed,  are  com¬ 
parable  rather  to  pillars.  It  is  accordingly  incorrect  to  speak 
of  the  sporangium  as  chambered,  for  the  fertile  cells  are  not 
segregated  into  loculi,  but  form  a  continuous  mass  pierced  here 
and  there  by  the  trabeculae.  It  is  hoped  that  figs.  44-47  will 
make  the  relations  of  the  trabeculae  clear.  The  shaded  por¬ 
tions  of  these  drawings  represent  the  fertile  regions,  and  the 
unshaded  portions  the  trabeculae  and  walls.  The  continuity  of 
the  sporogenous  mass  is  clearly  seen  in  the  tangential  section 

{fig-  46). 

A  more  detailed  account  of  the  development  of  the  micro¬ 
sporangium  will  now  be  given.  Fig.  48  shows  a  small  portion 
of  a  microsporangium  in  which  the  differentiation  into  sterile 
and  fertile  regions  has  just  begun.  The  fertile  cells  stain  deeply 
and  are  still  rapidly  multiplying,  as  is  evidenced  by  the  many 
karyokinetic  figures.  The  sterile  cells  have  almost  entirely 
ceased  divisions,  though  here  and  there  a  dividing  cell  may  be 
found.  It  is  important  to  notice  that  the  one  character  in  which 
the  two  regions  differ  is  in  the  relative  abundance  of  protoplasm, 
the  fertile  cells  being  densely  filled  with  deeply  staining  cyto¬ 
plasm,  while  the  cytoplasm  of  the  sterile  cells  is  beautifully 
vacuolated.  In  all  other  respects  the  cells  of  the  two  regions 
are  essentially  alike.  They  are  not  markedly  different  in  size, 


1900]  SP0R0PHYLLS  AND  SPORANGIA  OF  ISOETES  245 

or  in  the  size  and  appearance  of  their  nuclei,  nor  is  there  anything  in 
their  arrangement  to  suggest  a  difference  in  their  origin  or  growth. 
In  fact,  as  Professor  Bower  has  pointed  out,  there  is  here  a  most 
excellent  illustration  of  the  sterilization  of  sporogenous  tissue. 

The  trabeculae  at  this  age  show  about  15-25  cells  in  cross- 
section  (tangential  section  of  the  sporangium),  and  are  more  or 
less  cylindrical.  There  is  as  yet  no  tapetum.  Towards  the  outer 
and  inner  sides  of  the  sporangium  the  trabeculae  are  continuous 
with  about  three  layers  of  cells  which  form  the  sporangium  wall 
(fig.  49) .  That  the  trabeculae  and  walls  are  of  the  same  nature, 
both  being  the  result  of  sterilization  of  potentially  sporogenous 
tissue,  is  proved  not  only  by  the  similarity  of  their  cells,  and 
their  passing  uninterruptedly  into  one  another,  but  also  by  their 
relation  to  the  tapetum,  which  is  formed  out  of  the  layer  that 
lies  next  to  the  spore  mother  cells. 

The  inner  cells  of  the  trabeculae,  those  which  become  the 
trabeculae  proper  (i.  e.}  exclusive  of  the  tapetum),  are  at  first 
isodiametric  and  in  no  way  different  from  the  outer  ones.  But 
while  the  latter  are  undergoing  a  transformation  into  tapetum, 
the  former  undergo  changes  which  are  dependent  on  the  growth 
of  the  sporangium.  As  the  dimensions  of  the  sporangium 
increase  —  a  change  which  goes  on  rapidly  at  the  period  when 
the  sporogenous  cells  are  multiplying  —  the  trabeculae  are  nec¬ 
essarily  lengthened.  This  is  accomplished,  not  by  division  of 
the  cells,  but  merely  by  their  elongation.  At  the  same  time 
they  suffer  a  lateral  compression  from  the  growing  sporogenous 
cells  and  become  flattened  (fig.  50).  The  tabular  form  of  the 
cells  doubtless  furnishes  the  ground  for  the  common  view,  which 
ascribes  the  form  of  the  cells  to  the  direction  of  their  division 
planes.  Such  a  view  is  incorrect,  however,  for  divisions  have 
entirely  ceased  in  this  region  before  the  elongated  form  of  the 
cells  is  attained.  The  shape  of  the  cells  is  easily  accounted  for 
by  their  growth  in  the  one  direction  possible  for  them  while 
yielding  to  the  pressure  of  the  turgescent  mother  cells. 

In  this  connection  it  may  be  remarked  that  with  the  possible 
exception  of  the  tapetum  all  the  cells  of  the  sporangium,  after 


246 


BOTANICAL  GAZETTE 


[APRIL 


losing  their  power  of  division,  enter  upon  a  period  of  growth 
which  is  quite  comparable  to  that  occurring  in  vegetative  meri- 
stems.  The  difference  in  size  of  the  sporangia  represented  by 
figs.  43  and  63,  which  are  drawn  under  the  same  magnification, 
is  due  partly,  it  is  true,  to  increase  of  the  number  of  cells,  but  a 
glance  at  the  two  figures  shows  there  has  been  also  a  decided 
growth  of  the  individual  cells. 

Accompanying  the  modification  of  the  trabecular  cells,  there 
is  a  change  of  form  of  their  nuclei.  These  become  first  elon¬ 
gated  and  oval  (fig.  30),  and  finally  spindle-shaped,  suggestive 
of  the  changes  which  attend  the  development  of  the  vascular 
strand  out  of  the  tissues  of  a  growing  point.  Instances  of  much 
greater  elongation  than  that  shown  in  fig.  31  are  frequently  met 
with,  though  in  other  cases  the  changes  are  comparatively  slight. 
The  nuclei  at  this  time  are  relatively  large  and  prominent,  and 
appear  to  form  the  center  of  aggregation  of  what  little  cytoplasm 
still  remains  in  the  trabecular  cells.  In  old  sporangia  the  cells  of 
the  trabeculae  are  nearly  or  quite  empty,  and  much  compressed. 

Bower  has  discussed  the  function  of  the  trabeculae.  They 
may  serve  for  mechanical  support  of  the  sporangium,  or  to  afford 
a  larger  nutritive  surface,  or,  since  the  two  functions  are  not 
incompatible,  for  both.  The  relation  of  the  trabeculae  to  the 
base  of  the  sporangium  where  it  is  closest  to  the  vascular  bun¬ 
dle,  and  the  resemblance  of  the  nuclei  to  those  of  plerome  regions 
in  general,  suggested  to  me  that  the  trabeculae  might  be  the  chan¬ 
nels  through  which  nutriment  is  supplied  to  the  spores ;  but  the 
suggestion  is  not  borne  out  by  observation.  It  is  clear  that  in  a 
hydrophytic  plant  no  elaborate  apparatus  is  needed  to  provide 
the  sporangium  with  water,  which  can  easily  enter  directly  from 
the  outside ;  and  an  examination  of  my  sections  shows  that 
the  organized  food  stuffs,  such  as  starch  and  oil,  pass  to  the 
spores  through  the  inner  wall  of  the  sporangium,  and  not  through 
the  trabeculae. 

The  tapetum,  as  already  stated,  is  organized  out  of  that  layer 
of  the  sterile  cells,  whether  of  wall  or  trabeculae,  which  is  in  contact 
with  the  fertile  cells.  At  a  stage  between  those  shown  in  figs.  48 


igooj  SPOROPHYLLS  AND  SPORANGIA  OF  ISOETES  247 

and  50,  the  cells  of  this  layer  multiply  rapidly.  They  are  fre¬ 
quently  found  in  mitotic  division,  with  the  axis  of  the  spindle 
always  perpendicular  to  the  surface  of  the  trabeculae  or  sporan¬ 
gium  wall.  Divisions  may  still  go  on  here  after  the  spore 
mother  cells  have  reached  maturity,  and  the  changes  of  the 
trabeculae  are  nearly  complete.  In  this  way  the  tapetal  cells 
become  very  numerous,  but  reduced  in  size.  They  form  but  a 
single  layer  except  in  limited  areas,  where  a  doubling  may  some¬ 
times  occur. 

At  first  the  tapetum  is  not  deeply  stained  {fig.  50),  but  as 
the  spore  mother  cells  prepare  for  their  tetrad  division,  the 
tapetal  contents  increase  in  density,  and  they  continue  to  do  so 
until  they  surpass  young  spores  in  this  respect. 

From  what  has  been  said,  and  from  figs.  ^7,  etc.,  it  will  be 
understood  that  the  tapetum  completely  invests  the  trabeculae 
and  sporangium  wall,  forming  a  lining  layer  everywhere  between 
the  spore  mother  cells  and  the  sterile  regions.  It  is  a  persistent 
layer,  and  in  this  respect  is  to  be  contrasted  with  that  of  most 
ferns  and  angiosperms.  In  these  latter  the  walls  of  the  tapetum 
break  down  and  are  dissolved,  the  cells  become  disorganized, 
and  their  materials,  mingling  with  the  other  contents  of  the 
sporangium,  are  used  to  nourish  the  mother  cells  or  young  spores. 
In  Isoetes,  however,  as  in  Lycopodium  and  Selaginella,  no  such 
disorganization  of  the  tapetum  occurs.  Its  cells  do  not  fall 
apart  and  its  walls  are  not  absorbed.  In  old  sporangia  it  is  still 
recognizable,  though  often  its  contents  have  been  lost  and  the 
walls  are  pushed  nearly  together. 

Probably  the  tapetum  can  best  be  regarded  as  a  gland  or 
layer  of  glandular  cells.  If  so,  the  manner  of  action  in  a  per¬ 
sistent  tapetum,  such  as  that  of  Isoetes,  Lycopodium,  and  Selagi¬ 
nella,  must  be  quite  different  from  what  it  is  in  a  tapetum  which 
is  regularly  disorganized  and  absorbed.  In  the  one  case  the 
nutrient  substances  secreted  by  the  cells  must  be  passed  on 
through  the  walls  into  the  cavity  in  which  the  young  cells  are 
growing.  In  the  other  case  there  can  be  little  or  no  passing  of 
nutrient  substances  through  the  walls,  but  at  the  proper  time  the 


248 


BOTANICAL  GAZETTE 


[APRIL 


secreted  materials  are  rendered  available  by  the  total  collapse  of 
the  cells. 

In  many  plants  also,  especially  in  those  in  which  the  tapetum 
undergoes  complete  disorganization,  it  is  common  for  the  tapetal 
cells  to  become  multinucleate,  the  division  of  the  nuclei  being 
sometimes  accomplished  by  karyokinesis,  but  mostly  by  amitosis. 
The  cells  of  the  tapetum  of  Isoetes,  in  this  respect  again  agreeing 
with  Lycopodium  and  Selaginella,  are  uniformly  uninucleate. 

In  almost  every  sporangium  examined  the  number  of  layers 
of  cells  outside  the  fertile  regions  when  they  first  become  dis¬ 
tinct  is  three.  In  a  very  few  cases  there  were  four  layers.  As 
already  shown,  the  innermost  of  these  becomes  tapetum.  Of 
the  other  two  layers,  one,  apparently  the  hypodermal,  usually 
undergoes  division,  so  that  the  wall  region  ultimately  consists  of 
three  layers  outside  the  tapetum. 

At  the  base  of  the  sporangium,  between  it  and  the  vascular 
bundle,  are  a  few  layers  of  cells  which  may  be  regarded  as  the 
inner  wall  of  the  sporangium.  The  exact  origin  of  these  I  have 
not  been  able  to  make  out.  Whether,  like  the  outer  wall,  they 
are  derived  from  the  sterilization  of  sporogenous  tissue,  or 
whether  they  are  derived  from  the  tissues  underlying  the  original 
archesporium,  I  cannot  say.  It  is  always  difficult  in  all  sporangia 
except  the  very  youngest  to  define  the  exact  inner  limits. 
Between  the  vascular  bundle  and  the  three  or  four  outer  layers 
where  growth  and  division  are  most  actively  carried  on,  there  is 
a  mass  of  small  cells  staining  deeply.  Such  a  section  as  fig.  38 
makes  it  probable  that  all  the  tissues  between  the  parenchyma¬ 
tous  sheath  of  the  xylem  and  the  outside  arises  from  the  sporan¬ 
gium  Anlage ,  and  that  therefore  the  inner  wall  arises  also  by 
sterilization. 

The  formation  of  the  microspores  in  Isoetes  takes  place  in 
much  the  same  way  as  in  other  vascular  plants.  After  the 
fertile  regions  have  ceased  their  cell  divisions,  the  cells  and 
their  nuclei  pass  through  a  period  of  rest  and  enlargement. 
The  nuclei  especially  increase  in  size  and  become  rich  in  chro¬ 
matin.  At  the  same  time  the  cytoplasm  remains  dense  and  never 


1900]  SPOROPHYLLS  AND  SPORANGIA  OF  ISOETES  249 

shows  the  vacuolated  appearance  of  the  sterile  cells.  Shortly 
afterwards  the  mother  cells  break  away  from  the  tapetum,  which 
from  this  time  on  gains  in  density  and  apparent  activity.  The 
mother  cells,  at  first  in  a  continuous  mass,  soon  break  up  into 
smaller  and  smaller  groups  of  cells  by  the  enlargement  of  the 
cavity  in  which  they  float.  Finally  the  individual  cells  fall  apart 
and  round  up,  and  pass  rapidly  through  the  two  divisions  by 
which  the  microspores  are  formed. 

No  attempt  was  made  to  follow  closely  the  cytology  of  these 
divisions  because  it  was  found  impossible  to  make  any  satisfac¬ 
tory  observations  on  the  corresponding  divisions  of  the  mega¬ 
spore  mother  cells.  The  following  notes  may  however  be  of 
interest.  The  achromatic  figures  appear  to  have  a  polycentric 
origin,  and  the  chromatin  passes  through  a  synapsis  stage.  All 
the  nuclei  make  their  preparation  for  division  and  begin  to 
divide  almost  simultaneously,  and  this  notwithstanding  their 
immense  number.  It  is  possible  to  find  a  better  series  of 
karyokinetic  figures  in  a  single  sporangium  of  many  ferns, 
where  there  are  but  sixteen  mother  cells,  than  in  an  Isoetes 
microsporangium  where  the  mother  cells  number  three  or  four 
times  as  many  thousand.  This  1  think  may  be  regarded  as  an 
additional  proof  of  the  growth  of  the  sporangium  as  a  unit,  and 
not  as  an  aggregation  of  segments. 

In  the  majority  of  cases  the  two  divisions  are  of  the  type 
which  is  characteristic  of  cycads  and  monocotyledons,  and  has 
been  called  “successive;”  that  is,  the  first  division  of  the  nucleus 
is  followed  by  the  formation  of  a  cell  wall  before  the  immedi¬ 
ately  following  division  of  the  daughter  nuclei  {fig.  3j).  The 
spores  in  this  case  are  bilateral  and  may  have  their  nuclei  in  one 
plane  or  in  two  planes  at  right  angles  to  each  other.  But  it  is 
not  at  all  infrequent  to  find  the  divisions  of  the  simultaneous 
type ;  that  is,  the  first  division  of  the  nucleus  is  not  attended  by 
cell  division,  but  before  a  wall  is  formed  between  the  daughter 
cells  each  new  nucleus  begins  its  second  division  {fig.  34).  In 
this  case  the  spores  may  be  of  the  bilateral  type,  as  in  fig.  33 
a  and  b ,  or  they  may  be  tetrahedral  as  in  fig.  33  c.  Much 


250  BOTANICAL  GAZETTE  [april 

diversity  may  be  found  within  a  single  sporangium.  Figs.  53  a, 
b ,  c,  and  54  a ,  b,  c ,  were  all  taken  from  the  same  section  of  the 
same  sporangium.  Probably  the  variation  in  this  respect  is  not 
of  great  importance  except  as  indicating  that  the  divisions  of 
Isoetes  have  not  acquired  so  definite  and  settled  a  character  as 
those  of  most  other  plants. 

Although  the  nuclei  of  the  young  spores  may  arrange  them¬ 
selves  in  typical  tetrahedral  fashion,  there  is  an  important  differ¬ 
ence  between  their  relation  here  and  in  the  tetrahedral  divisions 
of  dicotyledons,  Lycopodium,  etc.  In  these  it  is  well  known  that 
all  four  nuclei  (of  such  a  stage  as  fig.  54)  become  connected  by 
spindle  fibers,  and  that  the  walls  separating  the  spores  are 
formed  in  connection  with  the  thickening  of  the  cell  plates  of 
the  six  spindles.  In  spite  of  careful  search  I  have  been  unable 
to  find  in  Isoetes  any  such  sextuple  spindles.  The  daughter 
nuclei  are  connected  only  in  pairs,  as  in  fig.  33  or  34.  In  what 
way  the  spore  walls  originate  in  such  cases  I  cannot  conjecture. 
It  seems  certain  they  are  not  formed  in  connection  with  the 
achromatic  figures,  unless  it  is  possible  that  the  cell  plate,  which 
is  always  present  in  the  first  division,  may  make  its  influence  felt 
later  on,  and  ultimately  serve  as  the  basis  of  the  wall. 

The  young  tetrads  soon  fall  apart,  and  the  individual  spores 
lose  their  angularity  and  round  up,  still  retaining  traces,  how¬ 
ever,  of  the  bilateral  shape  impressed  upon  them  by  their 
manner  of  origin.  When  once  the  permanent  form  is  assumed 
there  is  little  further  increase  of  size.  The  mature  spores  of 
fig.  36  are  little  larger,  it  will  be  seen,  than  the  newly  formed 
spores  of  fig.  51. 

An  interesting  phenomenon  in  connection  with  the  micro¬ 
spores  is  the  extreme  smallness  of  their  nuclei  in  comparison 
with  those  of  the  mother  cells.  One  would  naturally  expect  the 
relative  volumes  to  be  about  1 : 4,  or  the  relative  diameters  to  be 
about  3  :  5  (since  ^  \  =  f  nearly) .  But  the  volume  of  the  micro¬ 
spore  nucleus  is  really  no  more  than  one  twelfth  of  this  esti¬ 
mate  ;  or  to  express  the  comparison  in  another  way,  it  would 
need  the  nuclei  of  fifty  microspores  combined  to  equal  the 


1 9oo]  SPOROPHYLLS  AND  SPORANGIA  OF  ISOETES  251 

volume  of  one  mother  cell  nucleus.  Very  likely  similar  reduc¬ 
tions  in  the  volume  of  the  microspore  nuclei  occur  during  the 
tetrad  division  of  other  plants,  but  I  have  not  seen  any  other 
case  where  the  disparity  of  size  is  so  great,  nor  do  I  remember 
to  have  read  any  record  of  such  a  reduction. 

The  number  of  spores  formed  within  a  microsporangium  is 
enormous  —  much  greater  than  in  any  other  living  plant.  In  some 
species  it  is  said  to  exceed  a  million.  But  the  largest  number  I 
have  found  in  /.  echinospora  is  300,000.  My  estimates  place  the 
average  number  from  150,000  to  250,000. 

As  is  well  known,  no  provision  is  made  for  the  dehiscence  of 
the  sporangium  wall.  The  spores  are  set  free  only  by  the  decay 
of  the  tissues  enclosing  them. 

FURTHER  DEVELOPMENT  OF  THE  MEGASPORANGIUM. 

My  observations  on  the  development  of  the  megasporan¬ 
gium  differ  very  much  from  those  of  previous  investigators,  so 
very  much,  indeed,  that  I  would  be  loath  to  present  them  at  all 
had  I  not  confirmed  them  again  and  again  by  long  and  careful 
study.  These  differences  are  concerned  not  only  with  the  origin 
•of  the  archeSporium  and  early  growth  of  the  sporangium, 
which  have  been  already  spoken  of,  but  they  involve  also  the 
manner  of  selection  of  the  mother  cells  and  the  origin  and 
behavior  of  the  tapetum.  A  discussion  of  the  points  at  issue  will 
be  reserved  until  the  general  history  of  the  megasporangium  has 
been  considered. 

One  of  the  first  megasporangia  which  I  sectioned  presented 
the  appearance  shown  diagrammatically  in  fig.  6j .  The  two  large 
cells  M  and  M  are  evidently  megaspore  mother  cells,  but  what 
is  the  group  of  cells  a ,  corresponding  to  them  in  outline  and 
position  ?  It  consists  of  six  cells  in  all,  three  in  the  section 
under  examination,  and  three  others  in  the  adjacent  section.  A 
little  search  discovered  other  similar  groups  of  a  variable  num¬ 
ber  of  cells,  sometimes  but  two  or  three,  often  five  or  six.  If 
the  number  had  been  constantly  four  the  groups  might  have 
been  regarded  as  spores  resulting  from  a  precocious  division  of 


252 


BOTANICAL  GAZETTE 


[APRIL 


the  mother  cells.  But  that  explanation  being  precluded  it 
became  necessary  to  determine  their  relation  to  the  single  large 
mother  cells,  and  to  learn  their  later  and  earlier  history.  In 
attempting  to  do  so  I  have  become  convinced  that  a  very  large 
number  of  cells  are  potentially  megaspore  mother  cells,  that  a 
considerable  number  of  these  make  a  start  to  differentiate  them¬ 
selves  fully  from  the  sterile  cells,  but  that  comparatively  few 
are  finally  successful  in  reaching  the  large  size  and  well-nour¬ 
ished  condition  necessary  for  the  production  of  megaspores. 

The  changes  which  first  distinguish  the  megasporangium 
occur  relatively  earlier  than  those  which  mark  the  microsporan¬ 
gium.  In  the  latter,  as  we  have  seen,  the  first  change  is  the 
separation  of  certain  sterile  regions  from  the  fertile  cells  as  indi¬ 
cated  by  a  difference  in  cell  contents.  In  the  former,  however, 
changes  occur  at  a  considerable  time  before  there  is  any  possibil¬ 
ity  of  distinguishing  the  trabeculae.  When  the  megasporangium 
has  reached  a  stage  of  development  considerably  more  advanced 
than  that  shown  in  fig.  43,  a  change  is  discernible  in  many  of  the 
cells  which  form  the  third  and  fourth  layers  approximately. 
The  whole  sporangium  has  at  this  time  entered  upon  the  period 
of  enlargement  due  to  the  growth  of  the  individual  cells.  But 
in  fig,  63  it  is  clear  that  certain  cells  have  greatly  outgrown 
their  fellows.  Their  well-nourished  condition  is  attested  by  the 
density  of  their  cytoplasm  and  their  large  nuclei,  which  contain 
many  nucleoli.  All  these  enlarged  cells  are  engaged  in  the 
struggle  to  become  mother  cells.  Which  and  how  many  will  be 
successful  will  probably  depend  upon  their  holding  an  advan¬ 
tageous  position  with  respect  to  the  supply  of  nutriment,  pei- 
haps  also  to  their  having  obtained  an  earlier  start. 

It  does  not  always  happen  that  a  considerable  group  of  cells, 
enlarge  together.  Indeed,  it  is  a  comparatively  rare  case  when 
all  the  cells  of  the  third  and  fourth  layers  enlarge  to  any  con¬ 
siderable  extent.  Sometimes  the  enlarging  cells  are  in  more  or 
less  isolated  groups  separated  by  cells  of  ordinary  size.  Fig. 
64  shows  such  a  group  of  cells,  taken  from  the  side  of  a  spo- 
rangium. 


igoo]  SPOROPHYLLS  AND  SPORANGIA  OF  ISOETES  253; 

Quite  often,  too,  it  happens  that  one  cell  gets  the  advantage 
almost  from  the  beginning.  But  it  may  be  stated  as  the  rule 
that  there  is  a  selection  and  partial  enlargement  of  many  more 
cells  than  can  ultimately  become  mother  cells,  and  these  enlarg¬ 
ing  cells  belong  mostly  to  the  third  and  fourth  layers  of  the 
sporangium,  either  extending  continuously  across  the  sporan¬ 
gium  or  occurring  in  groups  separated  by  ordinary  cells.  That 
this  condition  is  associated  with  the  selection  of  megaspore 
mother  cells  is  proved,  I  think,  by  the  fact  that  enlarging  cells.,, 
comparable  to  those  of  fig.  6j ,  are  never  found  in  the  sporangia 
formed  late  in  the  season,  that  is,  in  those  which  are  to  bear 
microspores. 

What  becomes  of  the  defeated  cells  ?  This  is  a  difficult  ques¬ 
tion  to  answer,  for  since  there  is  so  much  variation  in  the  early 
condition  of  the  megasporangium  it  is  impossible  when  examin¬ 
ing  one  of  the  later  stages  to  tell  just  what  the  antecedent  con¬ 
ditions  in  that  sporangium  may  have  been.  From  the  frequency 
with  which  karyokinetic  figures  appear  in  the  cells  surrounding 
the  nearly  mature  megaspore  mother  cells,  it  seems  pretty  cer¬ 
tain  that  the  cells  which  have  been  left  behind  in  the  struggle 
simply  divide  until  their  products  have  the  general  size  and 
appearance  of  the  other  cells  of  the  sporangium.  If  the  enlarge¬ 
ment  has  not  gone  very  far  the  cells  retain  their  angular  con¬ 
figuration  ;  if  it  has  gone  further  the  cells  may  round  up  while 
exerting  a  considerable  pressure  on  those  adjacent.  So  I  inter¬ 
pret  the  group  c  in  fig.  6j. 

Fig.  66  will  furnish  a  good  illustration  of  the  behavior  of  the 
unsuccessful  mother  cells,  although  no  single  section  can  be  so 
convincing  as  a  series  of  them.  The  tissues  are  somewhat  con¬ 
tracted,  but  this  defect  does  not  hide  the  rounded  form  of  cer¬ 
tain  groups  of  cells,  and  their  marked  resemblance,  except  in 
being  multicellular,  to  the  mother  cells.  The  section  contains 
but  one  fertile  mother  cell,  the  one  labeled  m.  One  other  is 
situated  in  the  opposite  end  of  the  sporangium,  just  beyond  the 
limit  of  the  figure.  The  cell  a  is  undergoing  division,  the  mitotic 
figure  being  seen  in  the  adjacent  section.  An  interesting  fact 


254 


BOTANICAL  GAZETTE 


[APRIL 


which  goes  far  to  explain  the  division  of  the  groups  b,  b ,  is  the 
occurrence  in  adjacent  sections  of  larger  undivided  cells  (fertile 
mother  cells),  similar  to  m,  and  so  situated  as  to  be  almost  or 
quite  in  contact  with  the  dividing  groups.  Their  proximity 
accounts  for  the  failure  of  the  groups  b,  b ,  to  produce  spores. 
Some  of  the  smaller  and  less  rounded  groups  probably  repre¬ 
sent  mother  cells  which  suffered  an  early  defeat,  while  the 
larger  groups  represent  those  which  held  out  almost  to  the  last. 
Such  cases  as  these,  which  can  be  easily  duplicated  in  rapidly 
growing  sporangia  of  the  right  age,  are  conclusive,  it  seems  to 
me,  when  considered  in  conjunction  with  the  manner  of  growth 
of  the  sporangium,  to  show  that  the  fertile  mother  cells  are 
selected  by  their  advantageous  environment  and  not  by  any 
strict  morphological  position. 

The  fertile  mother  cells  increase  enormously  in  size  before 
dividing  into  spores.  Their  nuclei  maintain  a  proportionate 
growth,  and  their  cytoplasm  remains  dense  though  not  homog¬ 
eneous,  and  frequently  contains  grains  of  starchy  matter  and 
drops  of  oil. 

Notwithstanding  the  large  size  of  the  mother  cells  and  of 
their  nuclei  I  was  unable  to  make  any  detailed  study  of  their 
division.  About  the  time  when  division  occurs,  the  cells  seem 
to  be  peculiarly  liable  to  suffer  plasmolysis,  for  under  the  action 
of  the  fixing  agent  they  are  contracted  to  a  mere  fraction  of 
their  proper  volume.  When  sectioned  in  this  condition  they  are 
seen  to  lie  free  in  large  cavities  which  presumably  they  filled 
completely  when  living,  and  they  stain  so  intensely  that  it  is 
impossible  to  make  out  any  details  of  the  karyokinetic  process. 
I  have  not  once  had  the  good  fortune  to  see  karyokinesis  in  an 
uncontracted  megaspore  mother  cell,  although  the  corresponding 
phase  of  the  microsporangium  offers  no  technical  obstructions 
to  cytological  study.  The  liability  of  the  megaspore  mother 
cells  to  suffer  contraction  in  the  process  of  fixation  was  noticed 
by  Kienitz-Gerloff  (i)  and  other  investigators;  it  is  possibly 
associated  with  the  entrance  of  the  nuclei  into  the  synapsis 
stage. 


SPOROPHYLLS  AND  SPORANGIA  OF  1S0ETES 


255 


1900] 

The  young  megaspores  almost  invariably  have  the  tetra¬ 
hedral  arrangement,  as  in  fig.  5Q.  Occasionally  the  bilateral 
arrangement  is  found,  in  which  case  the  divisions  so  far  as 
observed  are  successive  (figs.  60 ,  61). 

The  further  growth  of  the  megaspores,  the  manner  in  which 
their  walls  are  laid  down,  and  the  storing  of  reserve  material, 
were  not  investigated. 

The  arrangement  and  subsequent  development  of  the  trabec¬ 
ulae  and  tapetum  of  the  megasporangium  offer,  as  is  to  be 
expected,  a  rather  close  homology  to  what  is  seen  in  the 
microsporangium.  The  trabeculae  are  formed  out  of  the  same 
kind  of  cells  as  compose  all  the  other  parts  of  the  young  sporan¬ 
gium.  I  do  not  discover  any  grounds  for  considering  them  the 
product  of  a  peculiar  kind  of  growth.  They  are  altogether 
unrecognizable  in  the  young  sporangium,  and  their  position 
when  first  outlined  seems  to  be  determined  by  that  of  the 
mother  cells.  Not  until  these  have  been  selected  and  con¬ 
siderably  enlarged  is  it  possible  to  distinguish  the  trabeculae, 
which  then  appear  as  feebly-staining  bands  extending  from 
front  to  back  across  the  sporangium  midway  between  the  fertile 
cells. 

The  cells  of  the  trabeculae  proper  undergo  the  same  process 
of  elongation  and  flattening,  attended  by  elongation  of  their 
nuclei,  that  has  been  described  as  occurring  in  the  microsporan¬ 
gium.  The  only  noticeable  difference  is  that  in  the  megasporan¬ 
gium  the  trabeculae  are  relatively  fewer  in  number  and  more 
massive.  For  example,  in  one  cas^,  an  exceptional  one,  I 
counted  400  cells  in  a  cross  section  of  a  trabecula,  whereas  in  a 
microsporangium  the  number  of  cells  in  a  cross  section  of  a 
trabecula  rarely  exceed  fifty,  and  is  oftener  under  twenty-five. 
This  is  only  another  way  of  saying  that  the  process  of  steriliza¬ 
tion  has  gone  much  further  in  the  megasporangium  than  in  the 
microsporangium.  The  total  mass  of  the  megaspore  mother  cells 
in  a  sporangium  is  only  a  small  fraction  of  that  of  the  combined 
microspore  mother  cells,  though  doubtless  the  total  volume  of 
the  mature  spores  in  the  two  cases  is  about  equal. 


2  56 


BOTANICAL  GAZETTE 


[APRIL 


The  tapetum  is  formed  in  this  case  also  out  of  those  layers 
of  the  sterile  cells  which  border  upon  the  fertile  cells.  No  doubt 
a  considerable  part  of  it  is  derived  from  the  unsuccessful  mother 
cells ;  but  as  these  are  the  homologues  of  the  trabecular  cells  of 
the  microsporangium,  being  merely  sterile  sporogenous  cells,  the 
homology  of  tapetum  and  trabeculae  in  the  two  sporangia  is 
complete.  The  only  difference  which  it  is  necessary  to  notice 
is  the  greater  abundance  of  the  tapetum  in  the  megasporangium. 
Instead  of  being  a  single  layer  it  is  several  layers  in  thickness 
{figs.  57,  58),  and  often  projects  into  the  sporangial  cavity 
in  the  form  of  irregular  papillae,  especially  from  the  base  of  the 
sporangium.  A  rounding  up  of  the  cells  immediately  about  the 
megaspore  mother  cells,  such  as  is  described  and  figured  by 
Goebel,  I  was  never  able  to  find. 

Though  the  megaspore  mother  cells  do  not  lie  in  contact 
with  one  another  as  the  microspore  mother  cells  do,  but  are 
isolated  in  groups  of  one  or  sometimes  two,  the  cavities  in  which 
they  lie  become  continuous  in  the  older  sporangia.  This  is 
brought  about  by  a  very  great  enlargement  of  the  cavities  after 
the  formation  of  the  spores.  The  enlargement  seems  to  be  due 
to  turgescence,  induced  probably  by  the  osmotic  activity  of  the 
substances  surrounding  the  spores.  It  cannot  be  accounted  for 
by  mere  growth  of  the  wall  cells,  nor  by  that  of  the  young 
spores,  for  these  do  not  completely  fill  the  cavities.  I  have 
computed  the  enlargement  of  the  megasporangium  after  all  cell 
divisions  have  ceased  to  amount  to  an  increase  of  three  or  four 
times  in  volume.  A  similar  change  of  size,  though  less  in 
extent,  occurs  in  the  microsporangium. 

If  the  preceding  account  of  the  development  of  the  spo¬ 
rangia,  especially  of  the  megasporangia,  be  compared  with  the 
account  given  by  Goebel  ( 1 )  and  Sadebeck  (  1) ,  it  will  be  seen  that 
the  differences  are  considerable,  and  of  much  theoretical  impor¬ 
tance.  According  to  these  writers  certain  cells  of  the  arche- 
sporium  divide  only  by  the  periclinal  walls  which  serve  to 
cut  off  the  primary  tapetal  cells.  In  these  no  anticlinal  divisions 
occur.  One  cell  of  each  of  the  rows  formed  in  this  manner, 


1 9oo]  SPOROPHYLLS  AND  SPORANGIA  OF  ISOETES  257 

apparently  the  innermost  one,  though  that  point  is  not  made 
clear  in  the  descriptions,  becomes  the  megaspore  mother  cell.1 
In  certain  other  archesporial  cells  divisions  take  place  in  all 
planes,  but  more  particularly  in  the  anticlinal  direction.  The 
products  of  these  latter  cells  give  rise  to  the  trabeculae.  Vines 
in  his  text-book  gives  nearly  the  same  description,  but  says 
that  the  archesporial  cell  from  which  the  megaspore  mother  cell 
arises  undergoes  but  a  single  division. 

If  the  assertion  be  correct  that  certain  archesporial  cells 
develop  only  into  trabeculae  and  certain  others  only  into  mother 
cells  and  tapetum,  it  is  clear  that  there  must  be  two  categories 
of  archesporial  cells,  one  set  destined  to  become  sterile,  the 
other  to  become  fertile ;  and  these,  although  indistinguishable  in 
appearance  and  size,  are  quite  unlike  in  their  mode  of  division 
and  growth  and  in  the  ultimate  fate  of  their  derivatives.  It  is 
impossible,  too,  to  escape  the  inference  that  the  megaspore 
mother  cells  are  already  determined  in  position  and  number 
when  the  sporangium  has  got  no  further  in  its  development  than 
to  the  differentiation  of  an  archesporium.  Further,  the  spo¬ 
rangium  must  be  regarded  as  compound,  each  fertile  archesporial 
cell  representing  a  separate  sporangium,  and  each  sterile  one  an 
imperfect  wall.  These  conclusions,  which  I  think  are  logical  and 
necessary  deductions  from  Goebel’s  description,  are  all  incon¬ 
sistent  with  the  development  of  the  sporangium  as  I  have  found 
it  in  I.  e chino sp ora. 

In  order  to  bring  out  the  points  of  contrast  more  clearly 
I  will  summarize  them.  I  do  not  find  any  difference  among 
the  archesporial  cells  either  in  manner  of  development  or  of 
growth.  I  find  no  flattened  tapetal  cells  overlying  the  megaspore 
mother  cells.  I  find  no  grounds  whatever  for  the  assertion  that 
each  archesporial  cell  follows  an  independent  growth,  or  that 
each  megaspore  mother  cell  represents  one  archesporial  cell.  I 
do  not  even  find  a  single  definite  hypodermal  archesporium 
which  can  stand  as  the  starting  point  of  the  inferences  above 
enumerated.  On  the  other  hand,  I  find  the  derivatives  of  all 

*See,  however,  Schenck’s  Handbuch  3  :  392. 


258  BOTANICAL  GAZETTE  [april 

the  archesporial  cells  dividing  in  various  planes,  and  blending 
indistinguishably.  The  sporangium  is  single,  not  multiple,  and 
the  megaspore  mother  cells  are  not  morphologically  predeter¬ 
mined  but  are  physiologically  selected  from  among  a  large  num¬ 
ber  of  potentially  sporogenous  cells. 

Though  the  certainty  of  the  matter  must  depend  upon 
observation,  it  may  be  pointed  out  that  the  number  of  mega¬ 
spores  has  a  bearing  upon  the  question.  A  megasporangium 
contains  from  150  to  250  megaspores.  If  we  take  200  as  the 
average,  it  represents  fifty  mother  cells,  that  is,  according  to  the 
current  view,  fifty  archesporial  cells.  To  this  we  must  add  at 
least  fifty  others  for  the  trabeculae,  giving  a  total  of  one  hundrc  d 
.archesporial  cells.  It  does  not  need  a  very  careful  examination 
of  I.  echinospora  to  demonstrate  the  impossibility  of  there  being 
so  large  an  archesporium,  for  when  the  sporangium  has  a  super¬ 
ficies  of  one  hundred  cells  it  is  far  past  the  archesporial  stage. 
It  is,  I  think,  absolutely  certain  that  each  archesporial  cell  gives 
rise  to  several  megaspore  mother  cells,  as  well  as  to  trabeculae 
and  tapetum.  In  the  microsporangium,  too,  the  trabeculae  alone 
outnumber  the  archesporial  cells  (cf  figs.  ji}  46 );  and  their 
extreme  irregularity  and  frequent  branching  and  anastomosis 
make  their  origin  each  from  a  single  cell  exceedingly  improbable. 

[To  be  concluded .] 


SMITH  on  ISOETES 


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LIBRARY 


BOTANICAL  GAZETTE,  XXIX 


PLATE  XV 


SMITH  on  ISOETES 


LIBRARY 

UNWrrTtfiu.iN0is. 


07 AN 1  CAL  GAZETTE,  XXIX 


SMITH  on  ISOETES 


LIBRARY 

UNIVERSITY^  ILLINOIS. 


BOTANICAL  GAZETTE,  XXIX 


PLATE  XVII 


SMITH  on  ISOETES 


muSB'**' 


BOTANICAL  GAZETTE,  XXIX 


PLATE  XVIII 


SMITH  on  ISOETES 


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BOTANICAL  GAZETTE,  XXIX 


PLATE  XIX 


SMITH  on  ISOETES 


UiiRr\iiY 

UNIVfcflWTY  rf  VU-VHOIS. 


BOTANICAL  GAZETTE,  XXIX 


PLATE  XX 


SMITH  on  ISOETES 


LIBRARY 

OF  THE 

UNIVERSITY  of  ILLINOIS. 


THE  STRUCTURE  AND  DEVELOPMENT  OF  THE 
SPOROPHYLLS  AND  SPORANGIA  OF  ISOETES. 

CONTRIBUTIONS  FROM  THE  HULL  BOTANICAL  LABORATORY. 

XVIII. 

R.  Wilson  Smith. 

(with  plates  xiii-xx) 

(i Concluded  from  page  258) 

THE  SUCCESSION  OF  SPOROPHYLLS. 

The  order  of  succession  of  the  sporophylls  is  subject  to  some 
variation.  It  is  not  at  all  uncommon,  especially  in  I.  Engel- 
tnanni ,  to  find  the  regular  sequence  interrupted  by  the  occurrence 
of  several  megasporophylls  among  the  microsporophylls.  Occa¬ 
sionally,  also,  sporangia  are  found  containing  both  megaspores 
and  microspores.  This  is  rarely  the  case  in  wild  plants,  though 
quite  common,  along  with  other  irregularities,  in  those  cultivated 
in  the  laboratory.  Some  plants  taken  in  December,  after  grow¬ 
ing  rapidly  for  seven  or  eight  months  in  the  laboratory,  had 
formed  only  megasporangia ;  some  others,  though  producing  a 
few  microsporophylls,  had  failed  to  bring  any  microspores  to 
perfection. 

The  sterile  leaves  of  I.  echinospora  differ  from  the  fertile  ones 
chiefly  in  their  smaller  size,  the  reduction  of  the  sheathing  base, 
and  the  absence  of  a  developed  sporangium.  They  remain  green 
throughout  the  winter;  while  the  sporophylls,  set  free  by  decay 
of  the  base  and  buoyed  up  by  the  gas  within  the  numerous  air 
cavities,  are  borne  away  by  currents  or  waves.  A  close  study  of 
the  sterile  leaves  almost  always  reveals  the  presence  of  aborted 
sporangia.  These  range  in  size  from  a  few  to  many  hundred 
cells  ;  they  are  often  of  irregular  shape  and  have  lost  their  pro¬ 
toplasmic  contents,  though  now  and  then  one  is  found  in  which 
a  few  spores  have  matured.  A  longitudinal  section  of  a  typ¬ 
ical  sterile  leaf  is  shown  in  fig.  62 ,  in  which  the  shaded  part 
1900]  323 


324 


BOTANICAL  GAZETTE 


[may 


represents  the  undeveloped  sporangium.  The  sterilization  of  the 
sporangium  does  not  affect  the  development  of  the  velum,  a 
fact  which  supports  Hofmeister’s  view  of  the  primary  separation 
of  velum  and  sporangium.  The  occurrence  of  aborted  sporangia 
on  so  many  of  the  sterile  leaves  shows  that  all  the  leaves  are 
potentially  sporophylls,  and  suggests  the  probability  that  Isoetes 
has  retained  a  more  primitive  form  of  the  sporophyte  than  any 
other  vascular  plant. 

HOMOLOGY  OF  THE  ARCHESPORIUM. 

The  term  “  archesporium  ”  was  first  employed  by  Goebel  (i), 
who  defined  it  as  a  cell,  cell-row,  or  cell-plate,  from  which  all 
the  spore-producing  cells  are  formed,  and  who  concluded  that  in 
all  sporangia  the  archesporium  occupies  a  hypodermal  position. 
Allusion  has  been  made  to  the  difficulty  of  accepting  this  con¬ 
clusion  in  such  a  case  as  that  of  Isoetes,  but  the  difficulty  is  not 
peculiar  to  Isoetes.  Bower  has  shown  that  in  several  pterido- 
phytes  (Selaginella,  Equisetum,  Lycopodium)  the  archesporium 
is  not  delimited  by  the  first  periclinals  of  the  outer  layer.  How 
shall  we  define  the  archesporium  in  cases  where  there  is  no 
single  hypodermal  layer  from  which  the  whole  mass  of  sporog- 
enous  tissue  is  derived,  and  to  which  the  term  can  be  correctly 
applied  as  required  by  etymology  and  definition  ?  We  must 
either  modify  our  conception  of  the  archesporium  or  abandon 
the  term  altogether  as  failing  to  express  the  facts.  It  appears 
to  the  writer  that  by  changing  our  notion  of  the  necessary  posi¬ 
tion  of  an  archesporium  we  could  not  only  avoid  this  difficulty 
but  would  also  be  enabled  to  make  a  more  consistent  compari¬ 
son  of  the  sporangia  of  seed-plants  and  pteridophytes  than  is 
possible  with  the  present  nomenclature. 

It  is  pretty  generally  recognized  that  there  is  no  true  epi¬ 
dermis  in  pteridophytes.  The  so-called  epidermis  is  physiologi¬ 
cally  but  not  morphologically  equivalent  to  that  of  seed-plants, 
for  a  true  epidermis  is  traceable  to  a  primary  layer  of  the  embryo, 
the  dermatogen,  which  is  distinctly  present  only  in  seed-plants. 
As  the  dermatogen  is  not  represented  in  pteridophytes,  unless  it 


1 900]  SPOROPHYLLS  AND  SPORANGIA  OF  ISOETES  325 

be  in  the  root-tip,  there  is  consequently  no  true  epidermis,  and 
the  part  which  performs  the  functions  of  an  epidermis  is  the 
outermost  layer  of  the  periblem  derivatives.  Thus,  while  sper- 
matophytes  have  three  embryonic  tissue  regions,  the  plerome, 
the  periblem,  and  the  dermatogen,  pteridophytes,  with  the  possible 
exception  of  the  root-tip,  have  only  the  two  first-named  regions. 

The  archesporium  of  all  spermatophytes  is  hypodermal.  In 
no  case  is  it  known  to  be  derived  from  the  superficial  cells.  The 
epidermis  is  from  the  beginning  distinct  from  the  inner  cells  of 
the  sporangium.  Though  there  may  be  periclinal  divisions  in 
the  superficial  cells,  as  in  gymnosperms,  many  Ranunculaceae, 
etc.,  these  occur  only  after  the  differentiation  of  the  archespor¬ 
ium,  and  the  cells  so  added  merely  increase  the  thickness  of  the 
wall  or  apex  of  the  sporangium,  but  never  become  part  of  the 
sporogenous  complex.  The  true  epidermis,  set  apart  at  a  very 
early  period  from  the  inner  tissue  of  the  embryo,  is  incapable  of 
producing  spore-forming  cells.  That  role  is  played  by  special 
cells  of  the  outer  layer  of  the  periblem. 

It  is  otherwise  with  the  “hypodermal  archesporium”  of 
pteridophytes.  This  is  cut  off  by  periclinal  divisions  from  a 
superficial  cell  in  the  case  of  ferns  and  Equisetum,  and  from  a 
group  of  such  cells  in  the  case  of  the  Lycopodiales  and  Isoetes. 

But  if  the  absence  of  a  true  epidermis  in  pteridophytes  and 
the  homology  of  periblem  with  periblem  in  all  vascular  plants 
are  conceded,  then  the  hypodermal  cells  of  spermatophytes  are 
comparable,  not  to  the  hypodermal  cells  of  pteridophytes,  but 
to  the  superficial  cells.  As  we  have  seen,  the  sporogenous  mass 
in  pteridophytes  can  always  be  traced  to  superficial  cells,  but  in 
spermatophytes  to  hypodermal  cells  and  no  further  ;  that  is,  in 
all  cases  the  function  of  giving  rise  to  spore-producing  cells  is 
localized  in  certain  cells  of  the  outer  layer  of  the  periblem. 

These  facts  serve  to  show  the  inconsistency  of  undertaking 
to  limit  the  archesporium  to  a  hypodermal  position  in  all  cases. 
The  cell  or  group  of  cells  whether  superficial  or  hypodermal,  to 
which  in  a  last  analysis  all  the  sporogenous  portion  of  a  sporan¬ 
gium  can  be  traced,  ought  to  be  called  the  archesporium.  The 


326 


BOTANICAL  GAZETTE 


[may 


change  of  nomenclature  which  I  would  propose,  therefore,  refers 
only  to  the  position  of  the  archesporium,  and  not  at  all  to  the 
meaning  of  the  term.  It  is  simply  this:  let  the  term  archespo¬ 
rium  continue  to  be  used  as  at  present  in  descriptions  of  seed- 
plants,  but  let  it  be  understood  in  the  case  of  pteridophytes  to 
signify  the  superficial  cell  or  cells  from  which  the  spore-forming 
tissue  is  derived.  In  this  way  the  difficulties  which  have  been 
pointed  out  will  all  be  met,  and  a  better  system  of  homologies 
can  be  made  for  sporangia  in  general.  The  archesporium  will 
always  occupy  the  same  position  relative  to  the  primary  meri- 
stematic  regions,  and  will  be  the  only  part  from  which  the  spore¬ 
forming  tissue  arises. 

The  nomenclature  here  proposed  can  be  more  easily  under¬ 
stood  by  reference  to  the  accompanying  diagrams.  Figs.  69  and 
jo  represent  two  early  stages  of  the  sporangium  of  a  common 
fern  (Pteris),  but  for  the  present  purpose  may  be  taken  as  typ¬ 
ical  of  any  pteridophyte  sporangium.  The  shaded  cell  ( a )  would 
be  regarded  as  the  archesporium  ;  it  divides  into  an  inner  fertile 
cell  (/),  from  which  all  the  spore  mother  cells  are  derived,  and 
an  outer  cell  (5)  which  gives  rise  to  a  large  part  of  the  sporan¬ 
gium  wall.  In  some  instances,  as  we  have  seen,  the  separation 
into  fertile  and  sterile  cells  is  not  accomplished  by  the  first  divi¬ 
sion.  In  such  cases  there  is  no  contradiction  of  terms,  since  all 
the  spores  arise  from  the  archesporium.  The  final  condition  is 
the  same  in  all  cases,  the  difference  consisting  simply  in  the 
earlier  or  later  sterilization  of  the  wall  region. 

Fig.  ji  represents  a  young  microsporangium  and  fig .  jj  a 
young  megasporangium  of  an  angiosperm.  The  outer  layer  ( e ) ,  the 
epidermis,  takes  no  part  in  the  formation  of  the  spore  producing 
cells  ;  a  is  the  archesporium,  which  usually,  as  in  pteridophytes, 
divides  into  an  outer  sterile  region  (5)  called  the  primary  tape- 
turn,  and  an  inner  fertile  region  (/)  called  the  primary  sporog- 
enous  cell  or  cells  [figs.  J2 ,  J4).  The  name  primary  tapetum 
was  given  to  the  sterile  region  to  express  its  supposed  function 
of  giving  origin  to  the  true  or  functional  tapetum.  Enough  is 
now  known  of  the  origin  of  the  true  tapetum  to  enable  us  to  say 


1900]  SPOROPHYLLS  AND  SPORANGIA  OF  IS0E7ES  •  327 

it  has  no  definite  relation  to  the  primary  tapetum,  and  that  in 
fact  the  term  “primary  tapetum”  is  a  misnomer.  The  true 
tapetum,  in  many  cases  at  least,  is  not  represented  by  any  mor¬ 
phological  structure  in  the  young  sporangium. 

Is  it  not  possible  that  the  cells  s  s  of  figs.  J2,  represent  the 
wall  layer  oi  fig.  70,  and  that  the  “primary  tapetum,”  in  addition 
to  the  protective  and  sometimes  nutritive  purpose  which  its 
derivatives  subserve,  has  also  a  phylogenetic  meaning  as  a  sur¬ 
vival  of  the  pteridophyte  sporangium  wall  which  has  been  in 
great  part  replaced  by  the  true  epidermis  ?  Such  at  least  is  the 
view  suggested  by  a  comparison  of  embryonic  organs  in  general, 
and  of  the  relations  of  the  primary  sporogenous  cells. 

RELATIONS  OF  THE  VELUM. 

On  the  question  whether  the  velum  has  any  homologue 
among  other  plant  structures  my  observations  do  not  furnish 
any  information.  It  has  been  compared  on  the  one  hand 
with  the  indusium  of  ferns,  and  on  the  other  hand  with  the 
integument  of  an  ovule.  The  possibility  of  the  latter  relation 
certainly  has  not  been  disproved,  but  the  evidence  for  it  is  so 
scant  that  it  must  remain  merely  an  interesting  suggestion.  As 
to  the  other  relationship,  it  ought  to  be  borne  in  mind  that  the 
only  ferns  which  can  be  at  all  closely  related  to  Isoetes  are  the 
euspoi angiate  families,  and  all  of  these  bear  naked  sporangia. 
The  indusium  appears  in  fact  to  be  a  special  organ  of  the  higher 
leptosporangiate  ferns,  without  representation  in  the  lower  fami¬ 
lies,  such  as  the  Osmundaceae  or  in  the  eusporangiates.  This 
absence  of  an  indusium  in  the  intermediate  orders,  and  the 
doubtfulness  of  the  homology  of  the  various  outgrowths  known 
as  indusia,  make  it  impossible  to  regard  the  velum  and  indusium 
as  more  than  homoplastic  structures. 

THE  AFFINITIES  OF  ISOETES. 

The  systematic  position  of  Isoetes  has  been  discussed  again 
and  again.  By  Linnaeus  it  was  placed  among  the  vascular  crypto¬ 
gams,  where  most  later  taxonomists  have  been  content  to  leave  it. 
During  the  first  half  of  the  present  century  it  was  most  frequently 


328 


BOTANICAL  GAZETTE 


[may 


grouped  with  the  Marsiliaceae  and  Salviniaceae,  chiefly  on  the 
grounds  of  their  heterospory  and  hydrophytic  habit.  DeCan- 
dolle  was  the  first  to  suggest  a  connection  with  Lycopodium.  In 
this  view  he  was  followed  by  Brogtiiart,  Endlicher,  Hofmeister, 
and  the  later  German  botanists.  A  summary  of  the  various 
relationships  which  have  been  assigned  to  Isoetes  was  given  in 
1888  by  Vines  (i),who  in  the  same  article  put  forth  the  opinion 
that  its  affinities  are  with  the  eusporangiate  ferns,  rather  than 
with  the  Lycopodiales.  More  recently  Farmer  (1)  and  Camp¬ 
bell  (4)  have  expressed  their  concurrence  with  this  disposition 
of  the  genus. 

Since  this  classification  has  been  retained  by  Vines  in  his 
Text-Book  of  Botany,  and  adopted  by  Campbell  in  his  Mosses  and 
Ferns ,  it  will  not  be  unprofitable  to  re-examine  the  evidence, 
with  the  purpose  of  seeing  what  light  can  be  thrown  upon  the 
subject  by  the  present  and  other  recent  investigations. 

In  any  discussion  of  relationships,  and  especially  when  there 
is  so  great  diversity  of  opinion  as  in  the  present  case,  the  con¬ 
clusion  is  likely  to  be  a  personal  one  merely,  dependent  on  the 
kind  of  evidence  which  the  examiner  holds  most  weighty,  rather 
than  on  its  absolute  nature.  There  are  some  general  principles, 
however,  to  which  everyone  will  probably  assent,  and  which 
ought  to  govern  one  in  estimating  the  relative  value  of  the  con¬ 
flicting  evidence  on  which  the  taxonomist  relies.  In  the  first 
place,  the  larger  the  number  of  characters  in  which  there  is 
agreement,  the  closer  is  the  relationship,  especially  if  the  charac¬ 
ters  are  such  as  are  known  to  have  great  taxonomic  value  in 
groups  related  to  the  one  under  consideration.  Of  single 
characters,  those  which  are  most  constant  are  of  most  value, 
even  though  we  are  not  able  to  detect  their  special  utility.  It  is 
generally  accepted,  too,  that  those  characters  which  appear  in 
the  embryonic  stages  of  an  organism  serve  best  to  mark  its 
wider  relationships,  as  of  class  or  family,  while  characters  which 
do  not  display  themselves  till  later  in  the  individual  life  are 
better  adapted  to  distinguish  the  near  relationships  of  species 
and  genus.  This  principle  applies  not  merely  to  the  organism 


1900] 


SPOROPHYLLS  AND  SPORANGIA  OF  ISOETES 


329 


as  a  whole,  but  quite  as  fully  to  the  embryonic  stages  of  its  dif¬ 
ferent  organs,  such  as  leaf,  root,  sporangium,  and  the  like. 

In  conformity  with  these  principles  it  is  proper,  in  the  deter¬ 
mination  of  natural  affinities,  to  place  great  emphasis  upon  the 
reproductive  parts,  for  such  parts  are  found  to  show  very  great 
constancy  in  their  form  and  occurrence.  The  sporangia  espe¬ 
cially,  and  the  form  and  arrangement  of  the  sporophylls,  have 
long  been  recognized  as  of  the  highest  importance.  Thus,  the 
classification  of  the  Filicales  is  largely  based  on  sporangial 
characters  ;  and  the  position  of  the  Salviniaceae  and  Marsiliacese, 
which  was  formerly  as  unsettled  as  that  of  Isoetes,  was  estab¬ 
lished  beyond  doubt  as  soon  as  the  development  of  the  sporan¬ 
gia  was  fully  understood. 

It  is  chiefly  on  the  basis  of  the  superficial  resemblances  of 
the  sporangia  of  Isoetes  with  those  of  Lycopodium  and  Selagi- 
nella  that  it  has  been  so  long  associated  with  them.  If  we 
enumerate  the  chief  differences  between  the  sporangia  of  Lyco- 
podiales  and  of  ferns,  we  shall  see  that  in  every  particular  Isoetes 
agrees  with  the  former.  While  the  Filicales  bear  numerous 
sporangia  on  the  dorsal  surface  of  the  leaf,  Isoetes  and  the 
Lycopodiales,  with  the  exception  of  the  Psilotaceae,  the  exact 
relation  of  whose  sporangia  to  the  leaf  is  still  in  dispute,  bear 
but  one  sporangium  to  a  sporophyll,  and  that  on  the  ventral 
surface  at  the  base.  Such  exceptional  forms  as  the  Ophioglos- 
saceas  and  Marsiliaceae  do  not  help  us  in  this  inquiry.  Though 
it  may  be  true  that  the  whole  sporangiophore  of  the  Ophioglos- 
saceae  is,  as  several  morphologists  have  suggested,  the  homo- 
logue  of  the  single  sporangium  of  Lycopodium  or  Isoetes,  the 
suggestion  is  so  hypothetical  in  itself  as  to  give  no  support  to 
any  view  based  upon  it.  It  is  only  in  the  position  of  the  sporan¬ 
gium  that  these  families  approach  Isoetes  ;  in  other  sporangial 
characters,  such  as  number  and  development,  they  are  like  other 
ferns. 

The  relative  age  of  the  leaves,  when  the  sporangial  rudi¬ 
ments  first  make  their  appearance,  is  of  considerable  significance. 
In  the  Filicales,  with  the  exception  of  the  heterosporous  forms, 


33° 


BOTANICAL  GAZETTE 


[may 


which  being  leptosporangiate  cannot  be  closely  related  to 
Isoetes,  the  sporangia  appear  late  in  the  history  of  the  leaf. 
There  is  an  enormous  development  of  the  midrib  with  its  con¬ 
ductive  tissues,  and  of  the  expanded  pinnae,  before  the  sporan¬ 
gia  are  recognizable.  Nothing  is  more  striking,  however,  than 
the  quickness  with  which  in  the  Lycopodiales  and  Isoetes  the 
rudiment  of  the  sporangium  follows  the  inception  of  the  leaf, 
which  when  the  sporangium  first  comes  into  view  is  no  more  than 
a  mere  papilla  of  undifferentiated  tissue,  without  a  sign  of  photo¬ 
syntactic  or  conductive  tissue. 

Still  more  far-reaching  is  the  agreement  of  Isoetes  with  the 
Lycopodiales  in  the  character  of  the  sporangium  rudiment. 
Goebel  (i)  in  his  celebrated  paper  of  1 880-1  classified  sporangia 
as  leptosporangiate  or  eusporangiate  according  as  they  arise 
from  single  cells  or  from  groups  of  cells.  Though  the  two 
classes  are  connected  by  transitional  forms,  such  for  instance  as 
the  Osmundacese,  in  which  the  sporangia,  though  always  classi¬ 
fied  as  leptosporangiate,  do  not  arise  strictly  from  single  cells, 
the  distinction  has  been  approved  by  all  later  morphologists. 
The  leptosporangiate  plants  make  a  well-defined  and  consistent 
group,  but  the  eusporangiates  comprise  very  diverse  forms, 
including  the  several  divisions  of  seed-plants,  the  Lycopodiales, 
the  Equisetales,  and  part  of  the  Filicales.  If,  however,  we  leave 
out  accessories,  and  turn  our  attention  entirely  to  the  essential 
part  of  the  sporangium,  that  is  to  the  sporogenous  tissue,  we 
find  a  distinction  which  has  the  merit  of  leaving  the  Filicales 
an  unbroken  group,  and  of  agreeing  closely  with  what  is  required 
by  a  consideration  of  other  characters.  This  distinction  pertains 
to  the  origin  of  the  archesporium.  The  spore-forming  part  of  the 
sporangium  of  Isoetes  and  Lycopodiales  can  be  traced  back  to  a 
number  of  cells  placed  transversely  to  the  leaf,  but  of  all  other 
pteridophytes  to  a  single  cell.  Is  not  this  distinction  as  valid  as 
that  which  pertains  to  the  origin  of  the  whole  sporangium?  If 
so,  it  tends  strongly  to  justify  the  inclusion  of  the  Lycopodiales 
and  Isoetes  within  a  distinct  group  set  apart  from  all  other  vas¬ 
cular  cryptogams. 


igooj  SPOROPHYLLS  AND  SPORANGIA  OF  ISOETES  33  1 

Certain  other  features  of  the  sporangium  of  Isoetes  find 
duplication  only  among  members  of  the  Lycopodiales.  In  all 
the  higher  leptosporangiate  ferns  there  is  an  elaborate  mechan¬ 
ism  for  the  bursting  of  the  sporangium  and  the  scattering  of  the 
spores.  This  device,  consisting  of  a  row  of  peculiarly  thickened 
cells  (the  annulus),  and  a  group  of  cells  which  form  an  easy 
place  of  rupture  (the  stomium),  is  very  rudimentary  in  the 
lower  leptosporangiates  (Osmundacese) ,  and  in  the  Ophioglos- 
saceae  and  Marattiaceae,  but  it  is  not  altogether  absent.  There 
is  at  least  a  predetermined  line  along  which  dehiscence  shall 
take  place.  The  elaboration  of  this  dehiscence  apparatus  is  one 
of  the  chief  peculiarities  of  the  higher  leptosporangiates.  When 
we  turn  to  the  Lycopodiales  and  Isoetes,  however,  we  find  posi¬ 
tively  no  contrivance  for  dehiscence,  and  no  vestige  of  an 
annulus  or  stomium.  The  sporangium  wall  is  simple,  and 
bursts  by  desiccation  in  Lycopodium  and  Selaginella,  and  by 
decay  in  Isoetes;  and  neither  method  can  be  regarded  as  a 
specialization. 

Another  analogy  has  been  brought  to  light  by  Bower’s  dis¬ 
covery  in  Lepidostrobus  of  certain  radiating  strands  or  processes 
in  the  sporangium  which  are  regarded  by  him  as  very  probably 
of  the  nature  of  trabeculae.  Since  the  relationship  of  Lepidostro¬ 
bus  to  Lycopodium  can  hardly  be  doubted,  there  is  here  a  point 
of  contact  with  this  group  of  plants  in  a  feature  in  which  other¬ 
wise  Isoetes  stands  alone. 

Again,  Selaginella  and  Isoetes  agree  very  nearly  in  the  man¬ 
ner  of  selection  of  the  megaspore  mother  cells.  The  unselected 
mother  cells  do  not  divide  at  all,  and  all  the  spores  resulting 
from  the  division  of  the  fertile  ones  as  a  rule  reach  maturity. 
In  heterosporous  ferns  all  the  mother  cells  divide  into  spores,  of 
which  but  one  becomes  a  megaspore.  The  contrast  may  be 
expressed  in  the  statement  that  the  megasporangium  is  differ¬ 
entiated  in  Isoetes  and  Selaginella  before  the  tetrad  division,  but 
in  heterosporous  ferns  not  until  after  that  division. 

The  persistence  of  the  tapetum  in  Lycopodiales  and  Isoetes 
is  a  character  to  which  no  great  importance  is  to  be  attached,  for 


332 


BOTANICAL  GAZETTE 


[may 

tapetal  characters  are  notoriously  variable.  Such  bearing  as  it 
has,  however,  is  in  harmony  with  what  may  be  inferred  from 
other  features  of  the  sporangium.  It  involves  no  disorganization 
of  the  cells,  no  multiplication  of  nuclei  except  as  related  to  cell- 
division,  and  no  mingling  of  naked  protoplasm  with  the  young 
spores. 

One  of  the  facts  which  Vines  advanced  as  an  argument 
against  the  usually  accepted  classification  of  Isoetes  is  the 
absence  of  a  strobilus,  the  characteristic  arrangement  of  the 
sporophylls  in  the  Lycopodiales.  He  contrasts  also  the  elon¬ 
gated,  slender,  branched  stem  of  Lycopodium  or  Selaginella  with 
the  short  unbranched  stem  of  Isoetes,  which  much  more  closely 
resembles  that  of  some  eusporangiate  ferns.  It  may  be  doubted 
whether  such  superficial  characters,  unless  accompanied  by 
internal  features  of  which  they  are  the  outward  expression,  have 
any  value  in  settling  the  relationship  of  distinct  genera  or  families. 
At  all  events,  their  usefulness  in  angiosperm  taxonomy  is  limited 
to  the  distinction  of  species  ;  they  would  be  of  no  use  in  decid¬ 
ing  the  family  to  which  an  undetermined  species  ought  to  belong. 
I  am  inclined  to  think  the  whole  plant-body  of  Isoetes  can  best 
be  explained  as  a  shortened  strobilus,  just  such  as  Lycopodium 
would  become  by  suppression  of  the  stem  and  axis,  while  allow¬ 
ing  a  normal  development  of  the  leaves  and  sporangia. 

The  most  obvious  diagnostic  character  of  the  three  groups 
of  pteridophytes  is  furnished  by  the  leaves.  The  leaves  of  the 
Isoetes  are  siri  generis,  and  afford  little  ground  for  associating  it 
with  any  one  group  rather  than  another.  Though  they  are  rela¬ 
tively  few  and  large,  as  is  the  case  among  ferns,  their 
unbranched  outlines  and  simple  tissues  show  an  analogy  with 
the  leaves  of  Lycopodiales  ;  while  their  peculiar  vascular  bundles, 
and  chambers,  and  diaphragms  remove  them  as  effectually  from 
either  group.  There  is  record,  it  is  true,  of  a  fossil  Isoetes  with 
a  branched  leaf,  indicating,  when  taken  in  conjunction  with  the 
sudden  reduction  of  the  vascular  bundle  just  above  the  ligule, 
the  possibility  that  the  present  form  of  the  leaf  may  be  a 
reduced  one  representing  a  more  complex  ancestral  type.  But 


1900]  SPOROPHYLLS  AND  SPORANGIA  OF  ISOETES  333 

we  must  admit,  so  far  as  mature  leaf  structures  are  concerned, 
that  Isoetes  occupies  an  isolated,  and  in  no  sense  an  intermediate 
position. 

The  testimony  of  the  young  leaves,  however,  is  not  so  neu¬ 
tral.  The  form  of  the  leaf  rudiments,  their  manner  of  growth, 
and  arrangement  about  the  axes  are  the  same  in  Isoetes  as  in 
Lycopodium  and  Selaginella,  and  quite  different  from  what  is  seen 
among  ferns.  The  difference  is  not  fully  expressed  in  saying 
that  in  one  case  the  leaf  originates  from  a  single  apical  cell,  and 
grows  by  means  of  it,  and  that  in  the  other  case  the  initiative  is 
from  a  group  of  cells.  The  leaves  of  ferns  are  distinctly  acroge- 
nous,  which  method  of  growth  gives  them  the  power  of  assum¬ 
ing  complex  forms  and  allows  the  successive  and  often  slow 
formation  of  stipe,  pinnae,  and  pinnules,  and  their  gradual 
unfolding.  A  leaf  which  grows  as  does  that  of  Isoetes  has  its 
power  to  assume  a  complex  form  limited  to  the  time  when  it  is 
meristematic  throughout  ;  as  soon  as  the  apex  becomes  perma¬ 
nent  tissue  the  outline  of  the  leaf  is  determined.  The  difference 
between  such  leaves  is  fundamental  and  far-reaching.  A  Lyco¬ 
podium  leaf  could  easily  attain  the  size  of  an  Isoetes  leaf  by 
retaining  the  meristematic  power  for  a  longer  time,  for  they  dif¬ 
fer  only  in  degree.  The  leaf  of  a  fern  could  become  like  that 
of  Isoetes,  or  vice  versa ,  only  by  a  radical  change  in  the  manner 
of  growth. 

The  similarity  of  the  leaf  rudiments  of  Lycopodium  and 
Isoetes  is  only  a  particular  instance  of  a  general  likeness  which 
extends  to  all  their  embryonic  organs.  We  have  already  seen 
how  this  is  true  of  the  sporangia  ;  and  it  holds  equally  good  for 
the  roots  1  and  stem  apex.  In  none  of  these  organs  is  there  ever 
an  apical  cell  or  any  concentric  segmentation  of  the  apices,  such 
as  are  characteristic  of  all  the  Filicales  and  Equisetum.  A  dif¬ 
ference  in  this  respect  in  the  case  of  apical-growing  organs,  like 
the  roots  and  stem,  may  not  lead  to  important  differences  in  the 
mature  structures,  as  the  variation  in  the  stem  apices  of  Selag¬ 
inella  suffices  to  show.  But  a  comparative  examination  of 

IVan  Tieghem  (i),  but  Bruchmann  (i)  entertains  a  different  view. 


334 


BOTANICAL  GAZETTE 


[may 


meristems  was  shown  by  Bower  to  possess  considerable  phyloge¬ 
netic  value,  jn  the  case  of  ferns,  and  to  lead  to  results  which 
agree  with  those  arrived  at  by  a  comparison  of  other  characters. 
The  fact  that  Bower  has  since  changed  his  view  with  regard  to 
which  type  of  fern  is  more  primitive  does  not  in  any  way  lessen 
the  value  of  his  previous  conclusions.  If  we  extend  the  series 
made  out  by  him  it  would  be  in  this  order  :  typical  leptosporan- 
giate  ferns,  Osmundaceae,  eusporangiate  ferns,  Selaginella,  Iso- 
etes,  and  Lycopodium.  In  this  connection  the  dichotomy  of  the 
roots  of  Isoetes,  Lycopodium,  and  Selaginella  ought  not  to  be 
overlooked. 

The  ligules  of  Selaginella  and  Isoetes  were  by  Goebel  made 
the  ground  for  grouping  the  two  genera  into  one  order,  the 
Ligulatae,  though  the  classification  was  recognized  by  its  pro¬ 
poser  as  merely  one  of  convenience.  In  the  former  part  of  this 
paper  I  have  made  a  comparison  of  these  organs,  and  expressed 
the  view  that  their  similarity  is  sufficient  to  demonstrate  their 
homology.  If  this  view  is  correct,  it  furnishes  additional  sup¬ 
port  to  the  relationship  of  Isoetes  and  the  Lycopodiales, 
especially  in  consideration  of  the  discovery  of  a  ligule  in  the 
vegetative  leaves  and  the  sporophylls2  of  Lepidostrobus,  another 
lycopodiaceous  plant. 

Turning  now  to  the  gametophytes,  we  notice  that  when  Vines 
suggested  the  connection  of  Isoetes  and  ferns,  it  was  supposed 
that  important  differences  existed  between  the  female  gameto¬ 
phytes  of  Isoetes  and  Selaginella;  but  the  later  and  more  com¬ 
plete  investigations  of  Heinsen  (i)  and  Arnoldi  (i)  have  demon¬ 
strated  their  close  resemblance.  The  diaphragm  of  the  female 
gametophyte  of  Selaginella  is  not  a  true  septum,  and  does  not 
arise  as  Pfeffer  (i)  supposed  it  did,  by  the  division  of  the  spore 
into  two  cells.  In  both  Isoetes  and  Selaginella,  the  free  division  of 
nuclei,  their  parietal  placing,  and  the  gradual  extension  of  cell 
division  from -the  periphery  to  the  center  of  the  spore  are  the 
same,  and  have  no  counterpart  in  the  germination  of  the  mega¬ 
spores  of  heterosporous  ferns.  The  gametophytes  agree  also  in 
the  absence  of  chlorophyll. 

2  Maslen  (i). 


1900] 


SPOROPHYLLS  AND  SPORANGIA  OF  ISOETES 


335 


Nearly  similar  evidence  is  furnished  by  the  male  gameto- 
phyte.  Though  Belajeff  (i),  to  whom  we  owe  the  most  exact 
investigation  of  the  subject,  says  the  male  gametophytes  of 
Isoetes  and  Selaginella  afford  little  ground  for  relating  the  two 
genera,  he.  has  shown  several  points  of  resemblance,  such  as  the 
separation  of  the  prothallial  (or  rhizoidal)  cell  from  the  single 
antheridium  by  a  cellulose  wall,  and  the  final  dissolution  of  the 
non-cellulose  septa  of  the  antheridium  wall,  so  that  the  sperma- 
tozoids  float  free  in  the  cavity  of  the  spore. 

Though  not  disposed  to  place  much  dependence  as  a  clue  to 
the  working  out  of  phylogenetic  relationships  among  hetero- 
sporous  plants  on  such  structures  as  archegonia  and  antheridia, 
which  must  necessarily  conform  more  or  less  in  shape  to  the 
space  in  which  they  are  confined,  I  find  some  interest  in  the 
fact  that  Isoetes  and  Lycopodium  are  the  only  genera  of  pterido- 
phytes  in  which  the  occurrence  of  more  than  two  neck  canal 
nuclei  has  been  reported,  and  that  in  Isoetes,  as  in  Lycopodium 
Phlegmaria  and  Equisetum,  the  plane  of  the  division  of  the  primary 
neck  canal  nucleus  is  at  right  angles  to  the  archegonium  axis. 

The  two  characters  which  stand  most  in  opposition  to  the 
inclusion  of  Isoetes  in  the  group  Lycopodiales  are  its  multicil- 
iate  spermatozoid  and  the  embryogeny  of  its  sporophyte. 
Campbell  has  very  properly  emphasized  the  similarity  of  the 
Isoetes  spermatozoids  to  those  of  ferns.  It  requires  only  a  brief 
survey  of  the  plant  kingdom  to  show  the  great  constancy  of  the 
form  and  behavior  of  male  cells  in  different  classes  of  plants. 
Consider,  for  example,  the  non-motile  spermatozoids  of  the  Flori- 
deae,  or  the  biciliate  spermatozoids  of  bryophytes.  Accord¬ 
ingly,  if  we  still  classify  Isoetes  among  Lycopodiales,  we  must 
admit  that  the  multiciliate  spermatozoids  make  an  exception  to 
a  constancy  which  is  otherwise  remarkable.  Unfortunately,  we 
have  only  Lycopodium  and  Selaginella  for  comparison,  and  are 
still  in  ignorance  as  to  what  the  gametophytes  of  the  other 
genera  may  have  to  tell  us. 

The  embryo  of  Isoetes  finds  its  nearest  approximation  in 
Botrychium3  though  the  resemblance  may  be  only  an  external 

3  Jeffrey  ( i). 


336 


BOTANICAL  GAZETTE 


[may 


one,  due  to  the  late  differentiation  of  the  embryonic  organs 
and  the  suppression  of  the  stem,  rather  than  to  any  deep-seated 
likeness.  However  that  may  be,  the  suspensor  of  Lycopodium  and 
Selaginella  is  a  positive  morphological  character  separating  them 
from  Isoetes.  Probably  the  embryos  of  Isoetes  and  Botrychium 
can  be  looked  upon  as  generalized  types,  the  specialization  taking 
the  form  in  ferns  of  a  very  early  demarcation  of  the  embryonic 
organs,  and  in  Lycopodium  and  Selaginella  of  a  suspensor. 

Two  other  possible  reasons  for  relating  Isoetes  to  ferns 
deserve  a  passing  mention.  Of  the  connection  between  the 
velum  and  the  indusium  enough  has  already  been  said  ;  and  of 
the  agreement  of  the  stems  of  Isoetes  and  Botrychium  it  is  suf- 
ficent  to  say  that  the  agreement  is  simply  in  the  fact  of  second¬ 
ary  thickening. 

The  claim  that  Isoetes  is  the  genus  of  modern  pteridophytes 
which  makes  the  closest  approach  to  angiosperms,  particularly 
to  monocotyledons,  gives  it  an  interest  quite  out  of  proportion 
to  its  numerical  representation.  It  is  not  clear,  however,  that 
the  claim  is  well  supported  by  facts.  Unquestionably  Isoetes 
and  Selaginella,  in  their  heterospory,  and  their  intrasporic  and 
reduced  gametophytes,  exhibit  features  of  life  history  which 
run  closely  parallel  to  that  of  seed-plants  ;  but  such  features 
really  foreshadow  monocotyledons  no  more  than  they  do  other 
seed-plants.  The  hypodermal  archesporium,  and  the  origin  of 
the  megaspore  mother  cell  as  the  lowest  of  a  row  resulting  from 
periclinal  divisions  of  an  archesporial  cell — two  points  which  my 
observations  disprove  —  would,  if  established,  be  as  strong  proof 
of  a  gymnosperm  as  of  an  angiosperm  attachment.  Some  facts 
distinctly  favor  the  gymnosperm  connection  ;  these  are  the  man¬ 
ner  of  germination  of  the  megaspore,  and  the  method  of  selection 
of  the  megaspore  out  of  a  large  mass  of  potentially  spore- 
producing  cells,  as  in  Cycas,  Callitris,  etc.,  to  which  may  be 
added  whatever  favors  the  relationship  of  Isoetes  with  the  Lyco- 
podiales. 

Professor  Campbell  has  shown  that  the  embryo  of  Isoetes 
bears  a  likeness  to  that  of  a  monocotyledon  in  having  a  lateral 


900] 


SP0R0PHYLLS  AND  SPORANGIA  OF  ISOETES 


337 


stem  apex  and  a  terminal  cotyledon,  and  suggests  a  comparison 
with  the  embryo  of  Alisma  for  instance.  The  resemblance  in 
form  is  undoubtedly  very  close,  but  we  ought  not  to  overlook 
some  equally  important  differences.  The  entire  absence  of  a 
suspensor  in  Isoetes,  which  has  been  brought  forward  as  an 
objection  to  its  close  affinity  with  Lycopodium,  militates  quite 
as  strongly  against  an  affinity  with  monocotyledons ;  and  the 
foot,  which  is  particularly  well  developed  in  Isoetes,  cannot 
be  said  to  have  any  clear  representative  in  monocotyledon 
embryos. 

In  general  habit  Isoetes  has  been  compared  to  some  grasses, 
rushes,  and  the  like  ;  this  is  a  mere  external  resemblance  in  one 
of  the  most  adaptive  features  of  plants,  and  not  supported  by 
internal  and  essential  similarities.  A  similar  objection  can  be 
raised  to  the  comparison  of  the  stelar  regions  of  Isoetes  and  of 
such  monocotyledons  as  Dracaena.  In  the  latter  it  is  true  there 
is  a  secondary  thickening  carried  on  by  means  of  an  extra 
stelar  “  cambium,”  but  this  cambium  merely  adds  parenchyma¬ 
tous  tissue  within  which  separate  vascular  bundles  are  organ¬ 
ized  ;  there  is  nothing  strictly  comparable  to  the  prismatic  zone 
or  central  xylem  cylinder  of  Isoetes.  Even  were  the  likeness 
much  closer  than  it  is,  the  peculiar  stem  of  Dracaena,  Yucca, 
etc.,  is  so  certainly  a  newly  acquired,  and  not  a  primitive  charac¬ 
ter,  that  it  affords  no  sound  reason  for  deriving  monocotyledons 
through  an  Isoetes-like  type. 

To  one  who  has  followed  this  discussion  thus  far  it  will  be 
evident  that  in  the  writer’s  opinion  the  balance  of  evidence  is  in 
favor  of  relating  Isoetes  to  Lycopodium  and  Selaginella  rather 
than  to  eusporangiate  ferns.  Of  course  the  facts  are  not  all  in 
hand  as  yet,  and  new  discoveries  may  materially  affect  the 
aspect  of  the  case.  The  facts  which  the  present  investigations 
have  brought  to  light  certainly  tend  in  the  one  direction.  The 
mode  of  origin,  development,  position,  and  general  characters  of 
the  sporangia,  the  development  of  the  leaf,  and  the  nature  of 
the  ligule  point  to  the  correctness  of  including  Isoetes  among 
the  Lycopodiales ;  while  the  form  of  the  spermatozcids  and 


338 


BOTANICAL  GAZETTE 


[may 


embryo  show  the  necessity  of  making  it  a  separate  family.  The 
Lycopodiales  so  constituted  comprise  six  genera  pretty  widely 
separated  in  morphological  characters,  as  from  the  antiquity  of 
the  group  one  might  naturally  expect.  But  the  extreme  differ¬ 
ences  are  not  greater  than  in  the  Filicales.  If  we  can  include 
Azolla,  Marsilia,  the  common  ferns,  Hymenophyllum,  the  Marat- 
tiacese,  and  the  Ophioglossaceae  in  one  group,  it  ought  not  to 
appear  inconsistent  to  include  Psilotum,  Lycopodium,  Phyl- 
loglossum,  Selaginella,  and  Isoetes  in  a  group  of  coordinate  rank. 
A  fuller  knowledge  of  the  three  little-known  genera  may  tend 
to  confirm  this  view,  especially  if  they  depart  as  widely  from 
the  remaining  genera  in  other  characters  as  in  general  habit 
and  sporangia ;  but  if  their  gametophytes,  spermatozoids,  and 
embryos  agree  very  nearly  with  Lycopodium  and  Selaginella,  it 
will  probably  be  better  to  make  of  Isoetes  a  fourth  group  ,of 
pteridophytes  equivalent  in  rank  to  the  three  now  universally 
recognized.  If  an  affinity  with  seed  plants  must  be  sought,  the 
evidence  points  to  a  connection  with  gymnosperms  rather  than 
with  monocotyledons, 

SUMMARY. 

1.  The  stem  apex  lies  at  the  bottom  of  a  funnel-shaped 
depression,  around  the  sides  of  which  the  leaves  are  arranged 
spirally.  This  depression  is  produced  by  the  expansion  of  the 
cortical  cells  of  the  stem  in  all  directions. 

2.  The  leaves  arise  as  crescent-shaped  bands  of  meristematic 
tissue.  At  first  the  basal  part  of  the  leaf  (the  sheath)  grows 
most  rapidly  ;  afterwards  the  region  of  growth  is  transferred  to 
the  part  above  the  ligule.  There  is  no  persistent  or  sharply- 
marked  zone  of  meristem.  The  whole  leaf  is  meristematic  at 
first ;  it  then  gradually  passes  into  permanent  tissue,  the  change 
beginning  at  the  apex  and  extending  gradually  downwards. 

3.  The  air-cavities  are  formed  out  of  four  longitudinal  bands 
of  cells,  which  after  losing  their  contents  and  power  of  multipli¬ 
cation  are  ruptured  into  transverse  partitions  by  the  growth  of 
the  other  parts  of  the  leaf.  The  size,  but  not  the  number  of  the 
air-cavities,  increases  with  the  age  and  growth  of  the  leaf. 


1900] 


SPOROPHYLLS  AND  SPORANGIA  OP  ISOETES 


339 


4.  The  ligule  originates  in  a  single  vesicular  cell  as  described 
by  Hofmeister.  The  mature  ligule  can  be  distinguished  into 
four  regions:  (1)  the  sheath  which  has  its  origin  in  the  lower¬ 
most  cells  of  the  young  ligule,  (2)  the  glossopodium,  (3)  a 
region  of  living  cells,  and  (4)  a  region  of  disintegrating  cells. 

5.  The  rudiment  of  the  sporangium  is  a  transverse  row  of 
superficial  cells  below  the  ligule  ;  the  upper  part  of  this  gives  rise 
to  the  velum,  the  lower  part  to  the  sporangium  proper. 

6.  There  is  no  definite  hypodermal  archesporium.  The  middle 
cells  of  the  sporangium  Anlage  are  the  first  to  undergo  periclinal 
divisions.  Additions  to  the  sporogenous  complex  are  made  from 
the  superficial  cells  of  the  sporangium. 

7.  The  general  direction  of  growth  of  the  sporangium  is  at 
right  angles  to  the  face  of  the  leaf,  with  a  slight  tendency  in 
young  sporangia  to  an  upward  direction.  The  cells  are  not  in 
well-arranged  rows  or  stratified  layers. 

8.  There  is  no  evidence  that  certain  of  the  archesporial  cells 
give  rise  to  trabeculae  only,  and  certain  others  to  mother  cells 
only.  The  trabeculae  and  megaspore  mother  cells  or  groups  of 
microspore  mother  cells  greatly  outnumber  the  archesporial  cells. 

9.  There  is  no  evidence  that  each  of  the  primary  cells  of  the 
sporangium  pursues  an  independent  growth.  On  the  contrary, 
their  derivatives  blend  indistinguishably. 

ic.  The  microsprangia  and  megasporangia  are  indistinguish¬ 
able  until  they  have  attained  a  volume  of  15,000-25,000  cells. 

11.  The  sporangium  becomes  recognizable  as  a  microspo¬ 
rangium  by  its  differentiation  into  irregular  deeply-staining  and 
feebly-staining  radial  bands.  The  deeply-staining  regions  after 
a  period  of  active  multiplication  become  the  mother  cells.  The 
feebly-staining  regions  become  the  trabeculae,  walls,  and  tapetum, 

12.  The  tapetum  is  organized  out  of  the  layer  of  sterile  cells 
adjacent  to  the  mother  cells;  its  cells  are  small,  densely  cyto¬ 
plasmic,  and  persistent. 

13.  The  middle  cells  of  the  trabeculae  become  elongated  by 
compression  and  growth ;  their  nuclei  also  become  elongated 
and  spindle-shaped. 


340 


BOTANICAL  GAZETTE 


[may 


14.  The  outer  wall  of  the  microsporangium  is  usually  four 
layers  thick,  the  innermost  layer  being  part  of  the  .tapetum.  The 
inner  wall,  that  is  the  cells  between  the  base  of  the  sporangium 
and  the  vascular  bundle,  is  probably  formed  by  sterilization  of 
cells  derived  from  the  primary  cells  of  the  sporangium. 

15.  The  divisions  of  the  microspore  mother  cells  may  be 
either  successive  or  simultaneous.  The  two  spindles  of  the  sec¬ 
ond  division  do  not  become  connected  by  secondary  fibers.  The 
microspores  are  usually  bilateral  but  sometimes  tetrahedral. 

16.  The  number  of  microspores  in  a  sporangium  is  150,000— 
300,000. 

\ 

17.  A  sporangium  first  becomes  recognizable  as  a  megaspo¬ 
rangium  by  the  marked  enlargement  of  many  or  most  of  the  cells 
of  about  the  third  and  fourth  layers.  All  such  enlarged  cells 
are  to  be  regarded  as  potential  mother  cells,  and  the  number 
of  them  which  succeed  in  producing  megaspores  is  probably 
dependent  on  nutrition.  No  tabular  tapetal  cells  are  cut  off  in 
connection  with  the  development  of  the  megaspore  mother  cells, 
nor  is  the  megaspore  mother  cell  the  innermost  of  a  row  of  cells 
formed  from  a  single  archesporial  cell  in  a  manner  comparable  to 
what  is  seen  in  the  ovules  of  seed-plants. 

18.  Many  cells  which  enlarge  almost  to  the  size  of  mature 
mother  cells  are  finally  unable  to  give  rise  to  spores,  but  divide 
up  into  smaller  cells  which  ultimately  form  part  of  the  tapetum. 

19.  The  trabeculae,  tapetum,  and  walls  arise  in  the  megaspo¬ 
rangium  as  in  the  microsporangium,  the  chief  difference  being 
the  greater  massiveness  of  the  single  trabeculae  in  the  former 
and  the  much  greater  abundance  of  the  tapetum. 

20.  No  details  of  the  division  of  the  megaspore  mother  cell 
were  obtainable.  The  megaspores  are  usually  tetrahedral  in 
arrangement,  but  occasionally  bilateral.  The  number  in  a  spo¬ 
rangium  is  150-300. 

21.  The  first  leaves  of  a  season  are  megasporophylls,  and 
these  are  succeded  by  microsporophylls.  There  is  occasionally 
some  irregularity  in  the  order  of  succession,  and  sometimes  a 
sporangium  is  found  which  bears  both  kinds  of  spores. 


1 900]  SPOROPHYLLS  AND  SPORANGIA  OF  ISOETES  341 

22.  The  sterile  leaves  in  a  majority  of  cases  have  aborted 
sporangia.  When  these  have  made  any  considerable  develop¬ 
ment  they  are  usually  found  to  show  the  characters  of  megaspo¬ 
rangia. 

23.  The  sporangia  after  all  cell  divisions  have  ceased  continue 
to  increase  in  volume,  apparently  by  the  osmotic  properties  of 
the  substances  surrounding  the  young  spores. 

24.  An  attempt  to  relate  the  change  from  megasporophylls 
to  microsporophylls  to  an  exhaustion  of  the  nutritive  cortical 
cells  formed  in  the  preceding  year  was  unsuccessful. 

25.  To  secure  a  more  consistent  nomenclature  it  is  proposed 
to  employ  the  term  archesporium  in  speaking  of  a  pteridophyte 
sporangium  to  designate  the  superficial  cell  or  cells  from  which 
the  sporogenous  tissue  takes  its  origin. 

I  am  indebted  to  Professor  John  M.  Coulter,  of  the  University 
of  Chicago,  at  whose  suggestion  the  investigation  was  under¬ 
taken,  for  helpful  suggestions  and  criticisms.  Acknowledg¬ 
ments  are  also  due  to  Dr.  C.  J.  Chamberlain. 

McMaster  University, 

Toronto,  Canada. 

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30  : 773-779.  789-801,  805-819,  821-833,  837-851.  1872. 

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217.  1892. 

Janczewski,  Ed.  (i)  Vergleichende  Untersuchungen  ueber  die  Entwick- 
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440-448.  1872. 

Jeffrey,  E.  C.  (i)  The  gametophyte  of  Botrychium  Virginianum.  Trans. 
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(2)  The  development,  structure,  and  affinities  of  the  genus  Equise- 
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1881. 

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Millardet,  A.  (i)  Le  prothallium  male  des  cryptogames  vasculaires. 
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Ann.  Sci.  Nat.  Bot.  VII.  18  :  113-256.  1894. 

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505.  513-524.  1874. 

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BOTANICAL  GAZETTE 


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Van  Tieghem,  Ph.  (i)  (et  Duliot)  Recherches  comparatives  sur  l’origine 
des  membres  endogenes  dans  les  plantes  vasculaires.  Ann.  Sci.  Nat.  Bot. 
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(2)  Traite  de  Botanique.  1891. 

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223-224.  1888-9. 

(2).  A  text-book  of  botany.  1895. 

EXPLANATION  OF  PLATES  X1II-XX. 

The  drawings,  except  Jig.  1,  have  been  made  with  the  aid  of  a  camera 
lucida,  and  all  have  been  reduced  by  photography  to  two  fifths  of  their  origi¬ 
nal  size.  Both  in  the  text  and  in  .  the  explanation  of  the  plates,  the  terms 
transverse,  longitudinal,  and  tangential,  when  used  to  describe  sections  of  the 
ligule  and  the  sporangium 'are  to  be  understood  as  indicating  the  planes  in 
which  the  sporophylls  were  sectioned. 

Fig.  1.  Base  of  microsporophyll,  inner  face  showing  the  sporangium  (V), 
the  velum  (v),  and  the  ligule  (/).  X  4. 

Fig.  2.  Radial  longitudinal  section  of  base  of  sporophyll ;  s,  v,  /,  as  in  Jig .  * 

1.  X  5. 

Fig.  3.  Longitudinal  section  of  apex  of  a  small  plant.  X  48. 

Fig.  4.  Longitudinal  section  of  apex  of  a  larger  plant  ;  x,  the  tracheids, 
p ,  the  prismatic  layer,  t,  the  leaf  traces,  m,  the  cambium  (semidiagrammatic). 

X  48. 

Fig.  5.  Part  of  the  prismatic  ring  as  seen  in  transverse  section  of  the 
stem  ;  x,p,  t,  m ,  as  in  Jig.  4.  X  160. 

Fig.  6.  Cortical  cells  from  the  neighborhood  of  the  cambium.  X  240. 

Fig.  7.  Cortical  cells  from  the  outer  region  of  stem.  X  240. 

Fig.  8.  Longitudinal  section  of  a  number  of  young  sporophylls  ;  the  spo¬ 
rangia  are  indicated  by  the  dotted  outlines.  X  48. 

Fig.  9.  Cross  section  of  a  young  leaf  above  the  ligule.  X  300. 

Fig.  10.  Cross  section  of  a  young  leaf  more  advanced  than  that  shown  in 
fig.  9  ;  the  position  of  the  future  air  chambers  is  shown  by  the  groups  of  nearly 
empty  cells  ;  the  small  circles  of  Jigs.  9  and  10  indicate  the  side  towards' the 
axis  of  the  plant.  X  300. 

Fig.  1 1.  Part  of  a  longitudinal  section  of  a  leaf  more  advanced  than  that 
of  Jig.  10 ,  showing  origin  of  the  air  cavities ;  p,  the  phloem,  x,  the  xylem. 

X  300- 

Figs.  12-13.  Transverse  section  of  leaf  with  the  vesicular  cell  from  which 
the  ligule  originates.  X  300. 

Fig.  14.  First  division  of  the  ligule,  sectioned  in  plane  a — a  of  Jig.  15. 

X  490. 

Fig.  15.  First  division  of  the  ligule  seen  in  radial  longitudinal  section. 

X  490. 


SPOROPHYLLS  AND  SPORANGIA  OF  ISOETES 


345 


1900] 

Figs.  16,  18.  First  division  of  terminal  cell  of  ligule  sectioned  in  plane 
a — a  of  fig .  15.  X  49°- 

Fig.  1 7.  The  same  sectioned  in  plane  b — b  of  fig.  15.  X  490- 

Fig.  19.  Tangential  section  of  older  ligule.  X  490. 

Fig.  20.  Transverse  section  of  ligule  of  same  age  as  that  of  fig.  19. 
X  490. 

Fig.  21.  Transverse  section  of  still  older  ligule.  X  300. 

Fig.  22.  Median  radial  longitudinal  section  of  base  of  half-grown  ligule  ; 
s,  the  sheath,  g,  the  glossopodium,  v,  the  velum.  X  490. 

Fig.  23.  Radial  longitudinal  section  of  base  of  mature  ligule  at  the  posi¬ 
tion  indicated  by  a — b  of  fig.  25,  showing  the  thickened  cells  of  the  velum 
and  leaf  adjacent  to  the  ligule.  X  48. 

Fig.  24.  The  same  in  I.  Engelmanni.  X  48. 

Fig.  25.  Transverse  section  of  ligule  and  leaf  at  the  position  indicated  by 
a—b  oi fig.  23.  X  30- 

Fig.  26.  Median  radial  longitudinal  section  of  young  sporophyll,  showing 
rudiment  of  the  sporangium.  X  490. 

Figs.  27-28.  The  same  more  advanced.  X  490. 

Fig.  29.  Transverse  section  of  young  sporophyll  with  sporangium.  X  490. 

Fig.  30.  Transverse  section  of  sporangium  more  advanced.  X  490. 

Fig.  31.  Tangential  section  of  young  sporangium.  X  490. 

Figs.  32-38.  Radial  longitudinal  sections  of  young  sporophyll  ;  fig.  37  is 
a  section  through  the  side  of  the  sporangium  of  which  fig.  36  is  a  median  sec¬ 
tion  ;  v,  the  velum,  g,  the  glossopodium,  s,  the  sheath.  X  490. 

Fig.  39.  Transverse  section  of  sporangium  more  advanced.  X  490. 

Fig.  40.  Transverse  section  of  leaf  with  sporangium  of  same  age  as  that 
o t  fig- 39-  X  490- 

Fig.  41.  Median  radial  longitudinal  section  of  a  sporangium  of  about  the 
same  age  as  the  last.  X  490. 

Fig.  42.  Longitudinal  section  of  side  of  sporangium.  X  490. 

Fig.  43.  Transverse  section  of  a  sporangium  still  older  but  in  which  the 
trabeculae  are  not  yet  recognizable.  X  490. 

Fig.  44.  Transverse  section  of  young  leaves  and  of  microsporangium  at 
the  time  of  the  first  differentiation  of  fertile  and  sterile  regions  ;  the  shaded 
portion  represents  the  fertile  region  ;  v,  the  velum,  /,  the  ligule,  f,  the  vas¬ 
cular  bundle.  X  48. 

Fig.  45.  Oblique  nearly  tangential  section  of  microsporangium.  X  48. 

Fig.  46.  Tangential  section  of  microsporangium.  X  30. 

Fig.  47.  Cross  section  of  sporophyll  and  microsporangium,  showing  the 
trabeculae,  tapetum,  t,  and  a  few  microspores  ;  v,  the  velum  (semidiagram- 
matic).  X  30. 

Fig.  48.  Portion  of  microsporangium  at  the  time  of  the  first  differentiation 
of  fertile  and  sterile  regions.  X  490. 


/ 


346 


BOTANICAL  GAZETTE 


[may 


Fig.  49.  The  same,  showing  portion  of  outer  wall.  X  490. 

Fig.  50.  Portion  of  an  older  microsporangium,  showing  differentiation  of 
sterile  regions  into  trabeculae  and  tapetum  (7).  X  490. 

Fig.  51.  Portion  of  trabecula,  tapetum  (/),  and  3'oung  spores  of  a  micro¬ 
sporangium.  X  490. 

Fig.  52.  Part  of  outer  wall  and  tapetum  (/)  of  nearly  mature  microspo¬ 
rangium.  X  490. 

Figs.  53-55.  Division  of  mother  cells  to  form  microspores;  fig.  jj  illus¬ 
trates  successive  division  ;  fig.  54,  simultaneous  division  ;  fig.  jj,  the  shape 
of  the  spores,  bilateral  in  a  and  b,  tetrahedral  in  c.  X  490. 

Fig.  56.  Young  microspores.  X  490. 

Fig.  57.  Cross  section  of  megasporangium  with  young  spores  and  tape¬ 
tum  (7).  X  48. 

Fig.  58.  Portion  of  a  trabecula  and  tapetum  (7)  of  a  megasporangium, 
X  490. 

Fig.  59.  Tetrahedral  arrangement  of  young  megaspores.  X  490. 

Figs.  60-61.  Successive  division  of  megaspore  mother  cells,  spores 
bilateral.  X  490. 

Fig.  62.  Median  radial  longitudinal  section  of  sterile  leaf  with  aborted 
sporangium  (shaded). 

Fig.  63.  Transverse  section  of  megasporangium  first  distinguishable  as 
such.  X  490. 

Fig.  64.  Part  of  transverse  section  of  a  megasporangium  with  a  group  of 
potential  mother  cells.  X  490. 

Fig.  65.  The  same  with  a  single  mother  cell.  X  490. 

Fig.  66.  Part  of  a  transverse  section  of  a  megasporangium  ;  for  explana¬ 
tion  see  text.  X  490. 

Fig.  67.  Diagram  of  young  megasporangium. 

Fig.  68.  Diagram  of  a  longitudinal  section  of  the  stem.  See  p.  228. 

Figs.  69,  70.  Early  stages  of  the  sporangium  of  a  fern.  Diagrammatic. 
a,  archesporium  ;  f,  fertile  sporogenous  cell  ;  s,  sterile  wall  cell. 

Figs.  71,  72.  Early  stages  of  a  microsporangium  of  an  angiosperm. 
Diagrammatic,  a,  archesporium  ;  e,  epidermis ;  s,  primary  tapetum  ;  /,  pri¬ 
mary  sporogenous  cell  or  cells. 

Figs.  73,  74.  Early  stages  of  a  megasporangium  of  an  angiosperm. 
Diagrammatic.  Letters  as  in  figs.  77,  12. 


